The original 2" x 6" Portass lathe in "Portalathe" form as advertised in 1922 and probably built from 1921. Notice the bed clamped between the wooden uprights and the simple compound slide rest bolted to the bed.

First Generation: Slab-framed Portass "Portalathe" and Bench Model variants:
Advertised for the first time during 1922, in the December 21st issue of  "Model Engineer" magazine by the "Heeley Motor & Manufacturing Co." (before their move to Buttermere Works), the original Portass-branded lathe was a 2 1/8-inch centre height by 6-inches between centres gap-bed machine - the "Portalathe" - at £3 : 15s : 0d. This first lathe (and indeed many subsequent ones) were of minimal specification with the spindle running directly in the metal of the headstock and carrying a 2-step pulley (arranged for drive by "gut" round-leather rope) and a 33/8-inch diameter faceplate. A bolt-on compound slide rest was used, that could be fixed in either two positions on the saddle, and tiny "0" Morse centres employed in both headstock and tailstock.  The compound slide was a primitive affair which, at first glance, seemed to work on two parallel bars in the familiar Unimat style.  In fact, other small lathes at the time (e.g. Patrick and Goodell-Pratt) had dual-bar slides, but the first Portass lathes used machined castings arranged so as to be open on the top, exposing the screw to dirt and swarf. Even the cross-slide and tailstock handwheels were as simple as possible, being turned from the solid and, needless to say, there was no micrometer dial on the cross-slide feed screw.  Alignment of tailstock and bolt-on slide-rest was achieved through a cast lug fitting into a longitudinal central slot milled down the length of the bed.  The top of the bed was planed flat, and the outside edges made presentable, but there were no V-edges with gib strips to take up play.
An important part of the Portass marketing strategy was to offer their lathes through the then highly-successful Sheffield-based Graves mail-order company both as straightforward Portass models or, though a nationally-distributed catalogue, as the Graves Model E plain non-sliding, backgeared NV Model G and non-backgeared NV Model F with illustrations showing variants of the early Portass slab-bed machines with their distinctive open top-slide. However, one machine, the NC Model G, was a backgeared, screw-cutting lathe with a proper V-edged bed and conventional top-slide. The connection with Graves is interesting, and Portass advertisements often mentioned not the Company's own address, but that of Graves: "Write to Dept. M.E. J.G. Graves Sheffield" It must be assumed that Graves were using their considerable marketing and financial background to support Portass in an attempt to reach the ever-increasing "Deferred Terms" market.
Continued below:

1922 Portass advertisement announcing the 21/8" lathe in both bench and "Portalathe" forms as the Model A with a single tool slide and the Model B with a compound rest. There was also a "De-lux" with backgear, screwcutting and a set-over tailstock. This lathe was also sold badged (with cast-in lettering on the bed) as the "Enox".
Continued:
Second Generation: Cantilever bed "Baby" and new "3-inch" screwcutting models:
In 1926 came a new departure with the simultaneous announcement in the Model Engineer Magazine for April 22nd of two lathes, each of which was a significant advance in its class: a 21/8-inch model - a machine to become popular as the Model A and Model B "Baby Portass" (and also sold badged as the Eclipse for the Sheffield James Neil Tool Company) and a new 3-inch swing (9" in gap and 12" between centres) backgeared and screwcutting machine  Presumably, to distinguish it from the soon-to-be-discontinued 2-inch models, one eighth-inch was added to the swing and the lathe was also used to replace the original "Portalathe" version. The carriage could now driven along the length of the bed by an overhung, square-thread 10 t.p.i. leadscrew with the saddle-to-bed adjustment gib strip arranged at the back - a location that was easy to engineer but one where a solid-metal to solid-metal sliding surfaces would have been preferable. The new 3-inch model - expensive at £10 : 10s : 0d - was, in effect, a scaled-up Baby Portass but equipped with a T-slotted saddle and, when fitted with screwcutting, a dog-clutch on the leadscrew. Again, Graves, with their bigger advertising budget and obviously close contacts with Portass, took the lead in promoting the new models and ran a quarter-page advertisement in the same issue of Model Engineer offering  the 21/8" Baby lathe for a six-shillings down payment, followed by nine monthly payment of the same amount, giving a total of exactly £3. The Portass name for the plain 21/8" lathe with a screw-feed tailstock barrel was the Model B, and this, if paid for in cash, was available for £2 : 5s : 0d. A careful trawl though the advertisements of the time appears to confirm that the original Baby Portass continued in production until some point in the 1930s, when it was displaced in popularity by the tiny "Adept" and "Super Adept" models made by Fred Portass, Stanley's brother. Fuller details of these lathes can be found in the Adept section of the Archive.Soon the 21/8-inch Portass was being sold by S. Tyzack & Son Ltd. as the "Baby Zyto" -although just "Zyto" was cast into the bed. Defying the logic of mass production, one version has been identified as identical to the Portass model but, unaccountable, with the headstock-bearing clamp screws arranged at the front rather than the rear of the casting - probably the first of many subsequent examples of small alterations made with the cosmetic rebadgings of Portass machines. Because the new screwcutting 3-inch was expensive - and obviously selling slowly - Portass developed the Baby lathe into a proper backgeared and screwcutting model and advertised it with a "de-luxe" a tag. This lathe shared the same tailstock and top-slide as the plan Baby model but in a search for economy the "Baby" bed pattern was modified and reshaped to accept the longer headstock (necessary to accommodate the backgear) and a mounting for the changewheel banjo bracket. The lathe was offered at a more modest £4 : 5s : 0d. but, although cheap at the time for a fully-specified machine, it was a primitive affair, requiring skill to coax  good results from and was advertised for only a few years from 1929 onwards. This lathe must also have sold only slowly (today it is very rare), and by the early 1930s would have compared poorly against more highly specified and rigid machines such as Zyto and 3-inch Pool and Randa models, though these were more expensive at between £8 and £9.  In the late 1920s Portass were becoming well known, and must have entered one of their more successful phases for now several firms, including the Sheffield-based machine-tool and engineering suppliers Gregory & Taylor Ltd. together with Zyto and Graves, were all offering Portass lathes, each obviously trying to outdo the other and probably ordering batches of similar models but all with slight (essentially money-saving or cosmetic) modifications. Competition in payment terms was also rife, with Gregory & Taylor Ltd. for example, advertising the same item on 5% terms compared to the 10% offered by Graves.  However, although Portass were happily supplying Gregory & Taylor with machines, their close ties to Graves resulted in them issuing an advertisement, on the 21st of November, 1929, where they countered by using similarly illustrated copy, on the same page, but declaring: "Deferred terms available through: J.G. Graves, Sheffield". From conversations with Sheffield people it is known that customers enjoyed going direct to the works to collect their machines, and no doubt having a peep at what went on in the factory.
Continued below:

A more complete J.G.Graves advertisement from April 1926 with prices unchanged from 1922 but mentioning the inclusion of the foot-motor with wall stand and its availability as a separate item - with the bench lathe for an additional 25 shillings.

Continued:
Third Generation: Curvilinear bed, double-foot and improved "3-inch Tyzack 'Zyto'" and  variants.
On 17 March 1927 the Tyzack company introduced what was to become their most popular model, the 'Zyto". Possibly confused press reports of the time indicate that there might have been (though only for a short time), two different machines: the B.T.C. (a company Tyzack had taken over in 1926) and an improved Portass 3" backgeared and screwcutting model. Although both were advertised by the company, they offered few details of the B.T.C.  - possibly suspecting it might not make the grade - and concentrated their efforts on the Sheffield lathe. The 3" x 12.5" gap-bed Portass was a double-foot machine (in essence the recognisable ancestor of the very successful Portass "Model S") and had its praises sung in Tyzack's full-page introductory advertisement (see below). Even allowing for the hyperbolae of the day, "The finest and cheapest lathe ever produced", it was an advanced machine with the spindle screwed a useful ¾" BSF and running in gunmetal bearings. Its main deficiency was absence of a compound slide and the use of a single, swivelling tool slide, although this was rectified on later versions by being made available as an extra-cost option.
Larger and smaller variants of this Portass-built machine would be sold not only by Portass themselves and  Tyzack but also by a host of other firms (see the Portass home page) for four decades.  It is remarkable that, just a year after introducing the 3-inch cantilever-bed lathes, Portass could find the time to design and introduce an alternative range of machines - although, to be fair, these did retain already-proved features such as the dog-clutch and top-slide unit. However, a major advance was the use of an "outrigger" foot at the tailstock; this, while it might have induced distortion when bolted to a less-than-flat surface, allowed the very light bed to be offered in a variety of lengths. Most impressive of all is that, while the changes produced a machine of similar specification, they were more competitively priced, being some £3 cheaper than the 1926 three-inch machine
Soon after the Improved Zyto was launched, Portass introduced yet another new 21/8-inch swing lathe - while the similar-sized original Baby was still in production. These economy lathes shared many mechanical components with the Baby, including the tailstock and the late-production Baby top-slide, distinguishable by its hard-edged appearance. Whether  intended to replace the Baby or not, the effect was to displace it, and the new lathe was promoted vigorously until the late 1930s. Photographs showing a plain version, sold by the London store Gamages in 1939 for a modest £1 : 17s : 6d can be seen here.  Gamages described the plain machine as having 10-inch centres, a 23/4" gap, and other dimensions similar to the Baby; however, the main differences were a significantly more rigid bed - suited to vigorous cranking when turning oversized work - and a change in leadscrew pitch from 10 to 12 t.p.i. Naturally, while the 12 t.p.i thread gave a lower-geared feel, and allowed a steadier drive to be maintained, it was frowned upon by knowledgeable model engineers, who knew it is was impossible to fit a dial where one division equals .001", or any other another useful decimal equivalent. Curiously, Portass also introduced a slightly scaled-up  21/2" version of this model, but they are hard to distinguish from their smaller brother (unless a ruler is handy), the only immediate clue being the slender proportions of the vertical section of the tailstock casting. Tyzack also sold this model - and possibly a screwcutting version, too -  with the bed carrying "ZYTO" prominently cast in large letters. 
To confuse matter, sales literature issued by Portass during the late 1930s shows two models sold direct from the factory (the sheet is reproduced below): the ordinary 21/8-inch machine at £2 :5s :0d with lever-action tailstock (or 10 shilling more with a screw feed) that retained most features of the earlier versions and another sold as the more expensive  "Model-de-Lux". The latter was a completely different machine and had no parts interchangeable with the other;  although it lacked screwcutting the De-lux was a well specified machine and ideal for light home use. Features included a set-over tailstock, compound slide rest driven along the bed by a leadscrew and a 3-step headstock pulley (for round-rope drive) carried on a spindle running in bronze bearings.
All versions of the tiny Portass sold well, under multiple guises, both before and after the War, and it is not uncommon to find them today - though the screwcutting version is less often encountered.  Many, bought by skilled owners, were considerably modified, and pressed into service for which they were never designed..

1938 and the last of the Portass publicity literature for the long-lived 21/8" lathes is issued. By this time a considerably improved "Model-de-Lux" had been introduced, a machine that had no parts in common with the ordinary type

13th of December, 1923 and Graves advertise the first of their Portass-built "Portable" lathes with a "provisional patented" twin-bar slide-rest assembly. Although the advertisement fails to mention the fact, the operator can be seen in the act of pedalling a "foot-motor", a simple cast-iron flywheel assembly with grooves cut in its rim to provide a drive to the lathe via a round leather "gut" rope. The unit is made clear in an illustration below. The total cost, on "easy-terms", was £4 : 10s : 0d

February 1925 and Graves advertise their "Portalathe" - but shown as a bench machine. This was much cheaper than the real "Portalathe" with the deposit and payments totalling just £2 : 18s : 6d

A Mk. 2 version of the first Portass "Portalathe" - still using the same  simple "wall-leaning" stand but with its own bed feet instead of wooden side supports. This illustration is from the Graves catalogue and was described thus: "  ...designed to be propped against any wall without fixing, and may be carried as easily as an ordinary pair of folding steps." This was the first advertisement to acknowledge the presence of a "foot-motor" drive assembly. The version illustrated is the plain-turning model without backgear or screwcutting.

The "Baby" Portass with art-worked Zyto markings. With a 2.125" centre height and 10" between centres this was an attempt by Tyzacks to add a really simple little lathe to their range but, during the 1920s and 1930s, it was in competition with many others and, despite costing only £2: 7 : 6 against the £7 or £8 of its bigger (Zyto) brother,  few can have been sold and today is a rare find, though more common than the Eclipse.

The first Portass backgeared and screwcutting lathe as offered in 1926 through the Sheffield mail-order company J.G.Graves. Priced at a rather expensive £10 : 10s : 0d. this was, in effect this was a scaled-up "Baby Portass" and included a dog-clutch on the leadscrew and a T-slotted boring table. One significant change from the ordinary machines was the use of Drummond-like draw-in split headstock bearings - where the clearance was closed up as the tapered outside of the bearing was compressed by being pulled into its seating by a pair of set-screws.

Above (in black and white and below two versions in colour) a circa 1926 21/2-inch backgeared and screwcutting Portass developed from the "Baby" Portass. While having the same generic appearance as the plain-turning model little on this version is the same with even the cross slide and tailstock handwheels being different and made from castings, rather than turned from the solid. However, to save money, it is likely that the bed was cast from a modified Baby lathe pattern. The lathe was offered at a modest £4 : 5s : 0d. (compared to £10 : 10s : 0d for the 3-inch model) but, although cheap at the time for a fully-specified machine, it was a primitive affair, requiring skill to coax  good results from and was advertised for only a few years from 1929 onwards. Just visible in the picture - but shown more clearly below on another lathe with slight differences - are the Drummond-like tapered headstock bear

A third example, not quite complete but good enough to show the tapered bronze bearings as used on these very early backgeared and screwcutting Portass lathes

Bought new for £6 in the early 1930s, the first owner of this simple plain-turning, non-backgeared Portass may have been handicapped by a lack of money, but not skill, and set about making a number of significant improvements. Needing to use the lathe for horological and other precision work, first to go was the original spindle, replaced by one able to accept to draw-in 8 mm collets, with the tailstock barrel similarly modified to take 6 mm collets and controlled by a neat, lever-action unit that replaced the hard-to-use and insensitive screw thread.. A swivelling top slide was constructed and both it, and the cross slide, fitted with larger "balanced" handles and micrometer dials. The leadscrew was equipped with a large handle at the tailstock, a fitting that allowed a particularly steady and even movement to be applied to the cutting tool.