The earliest type of flat-bed 3.5" centre height Drummond had a distinctive headstock with pinch bolts passing beneath the bearings and cast-in bracing struts on the face between the bearing uprights. The first beds were of light construction with the short section of underside at the headstock end almost parallel to the ways and at the tailstock end rising so close to the top surface that it exposed, in side elevation, a good deal of the power shaft that lay beneath the leadscrew. Pictures of this lathe here

Continued:
In 1906, having presumably settled into their new premises, further changes were made to the lathe's design and the maker's publicity material made much of both the new headstock spindle bearings (previously explained) and the improved bed casting that, being considerably thicker and more heavily constructed, was much stronger than the original. The two beds are easily distinguished with the earlier having its underside edge parallel to the ways at the headstock end and, at the other, rising in a graceful arch to become very close to the top surface (exposing both top and bottom surfaces the shaft drive beneath the leadscrew) before looping down to form a vertical flat mounting surface for the leadscrew bracket. The new bed had a distinctive downward curl at the headstock end and a rather deeper wall at the tailstock end - these features being an easy way to determine from a photograph if a lathe really is one of the rare surviving first examples.  Sadly, the elegant hardwood (or horn) knob on the backgear shaft disappeared to be replaced by a tapered, one-piece steel handle - and an altered design of leadscrew clutch, with two positions instead of three, was introduced. The tailstock was modified with a longer bracket extending from the base casting and the loose clamp, instead of being machined from a block of steel, was now made of cast iron; unfortunately, the convenient locking handle was omitted and replaced by a nut that required the application of a loose spanner. The simple knock-in barrel-guide pin was moved from the side of the casting to the top; photographs of a circa-1906 model with some of these features can be seen here.
Changewheels ran directly on plain studs and were retained by circular, knurled-edge nuts. The mounting  bracket was L-shaped, a design foible left over from Victorian times that made it difficult to set up a very fine feed to the carriage drive. Changewheels supplied as standard consisted of: 20t, 30t, 35t, 38t, 40t, 45t, 50t, 55t, 60t and 65t. To generate metric threads an additional 63t changewheel could be purchased at extra cost.
In about 1907 the largely ornamental bracket beneath the leadscrew dog-clutch handle was omitted (it was held in place by two square-headed bolts) and the single, swivelling top slide modified to reduce the bending moment by bringing the clamping stud and nut in line with its slideway. As a final touch, the base of the slide was given a set of inscribed degree graduations. It was 1908 before a compound slide rest was finally introduced - initially with round handwheels and then with the "cranked" type, a design that was to become a Drummond 'trade-mark'.  Although some of the above changes can be found in contemporary advertisements, many cannot and numbers of lathes have been found where the specifications overlap and the year of production is uncertain. For example: lathes with a graduated base to the top slides, but still with the dog-clutch lever bracket in place; or a compound slide rest on a lathe with the older-style bearings. It is also entirely possible that the factory used up obsolete parts before a modification became permanent. Alternatively, customers may have ordered a cheaper specification or returned them to the factory for repair, alteration or updating; or rebuilt and adapted them over the years using whatever later parts came to hand.
Able to be swivelled on the bed the headstock could be combined with the fully-adjustable tool slide to allow the easy cutting of short tapers and tapered threads. However, it took until 1912 before the factory realised that fitting a locating dowel would help their customers in the important task of return the setting to zero. The gap in the bed could accommodate a piece of material a little over 9.5" in diameter and 3" thick and it was possible to set the tailstock over for long, fine-taper work. Supplied with the lathe as standard were: a faceplate, a small-angle plate with vertical T slots, catchplate, travelling steady, hand rest, two No. 1 Morse taper centres, a drive belt and spanners.
Although usually illustrated mounted on a self-contained treadle-and-flywheel stand, various kinds of wall and ceiling-mounted countershaft units were also available for those lucky enough to have either a stationary engine or even (though at great cost) an electricity supply and an electric motor. Although in its first incarnation this was a thoroughly well-specified product, it was substantially improved in 1912 and reached its final version through the two major rethinks of 1921 and 1925. It continued in production until 1942 when, overwhelmed with war-time demand for their high-speed production lathes, Drummond were forced by the Ministry of Supply (who controlled all material and machine-tool allocations) to pass the rights over to Myford. Advertised as the "Myford M-Type" it became known in model engineering circles as the "Myford-Drummond M-Type" - a general history of the machine can be followed on the Drummond/Myford M Page.
A set of detailed photographs of a particularly fine and original early 31/2" lathe can be seen here and another lathe, still in regular use in Germany (and with just two owners) here.
Other interesting very early examples of the 3.5-inch flat bed can also be seen here and here and here.
Surprisingly, having regard to the popularity of these early Drummond lathes and their domination of the top end of the amateur market, it is surprising that no third-party appears to have made any effort to market accessories - though there was the exception of a powered shaper, driven from the headstock spindle, that attached to the bed - though this was a unit that could be supplied with fittings for a variety of other contemporary models. Finally - how many were made? With all productions records lost the only evidence comes from a comment in a magazine article of 1921 quoting: Fifty of the new type 31/2-in lathes are made each week, and nearly that number of the popular 4-in (Round-bed) model". Extrapolating from these figures it would seem prudent to made some reductions: from 50 to 45 to allow for monthly fluctuations, down to 40 as compensation for slow sales in the early years; poor trading during the 1929-1931 depression and a further reduction approaching the war in 1939 as well as during the run out to the last examples dated 1951. Shall we say, 90,000?

The same early model as in the photograph above but suspected to have come from the second-edition catalogue showing a heavily art-worked picture disguising the change in the headstock casting and bearings--but not the bed  which is shown as the earliest type with the short length of underside at the headstock end finished in a section parallel to the ways and at the tailstock end rising to become very close to the ways before looping down to form a flat vertical surface for the leadscrew bracket. It is likely that the new headstocks were fitted for a while to earlier pattern beds--though for how long is impossible to say. The simple, wooden packing pieces being used to mount the motor-cycle cylinder might seem an amateurish effort but was (and remains) a highly effective way of holding a fragile component without damage. The pin that limited the travel of the dog-clutch lever (it can be seen protruding through the bed immediately below the gap) was quickly dropped - as was the unnecessary support plate itself in 1907.

A 1907/8 model showing the optimum metal removal capability when using the proper tool on the right material at the correct speed - and with a delicate rate of feed, skilfully applied. Although most pre-1912 lathes had a 3-step headstock pulley for drive by a 1/4" round leather (gut) drive examples are known of machines with 2-step pulleys that took a much larger belt; though not mentioned as an option in any known catalogue, these lathes would undoubtedly have been intended for tackling heavier work.

Here the headstock is seen set over and a short taper being turned. The round leather rope drive from the rim of the foot-operated treadle drive allowed for a certain degree of twist in its drive. The sliding backgear was fitted with an elegant horn or hardwood handle.

A 1908 Model with the new-for-that-year compound slide rest - note that the "support plate" bracket under the leadscrew dog-clutch lever is no longer fitted and the more substantial bed in comparison with the machines illustrated above - most obvious being the increase in depth between the bed ways and the top of the radius at the tailstock end and the down sweep of the casting at the headstock end.