The Model S was the smallest and cheapest of the Portass backgeared, screwcutting range and was made in considerable numbers both before WW2 and from 1951, when it was re-introduced. Although there would have been room in the range to give the various adaptations of the Model S a different names, or at least allocate them model numbers, just like the company's determination to use "Dreadnought" to describe all their larger lathes the term "Model S" was used for machines with a variety of capacities and specifications. Over the years at least four versions were offered with centre heights of 3, 3 3/8 and 35/8 inches and between-centre capacities of 121/2 and 18-inches.  Some Model S lathes were fitted with compound slide rests, others made do with a single, swivelling top slide. The first model illustrated below, one of the last made (in the mid 1950s), had a redesigned bed with vertical stiffening ribs cast into the section of the bed beneath the gap, a larger spindle and bearings, a compound slide rest with a thinner top slide. The dog-clutch lever was straight and pointed downwards - rather than being dog-legged and set upright as on earlier machines. An even more heavily modified version was also produced, although it is impossible to say if it was manufactured simultaneously with other models (and built at the request of a retailer) or just a more highly-developed version with the main improvement centred around a much stiffer headstock with the bearings braced by raising the front of the casting until it was level with the bearing clamp slits.
Many firms are known to have bought in Portass lathes and badged them as their own including: Altona, A.T.M., B.I.L., Bond's Maximus, "Eclipse" (for the Sheffield hand-tool makers James Neil & Sons) Excell, G.A. (George Adams), Gamages , Graves, James Grose Ltd. of London (the latter chiselling the Portass name off and substituting their own badge), Juniper, Randa, Temmah, Woolner and Zyto, All appear to have been based on established Portass models, nearly always the venerable "S Type", although in every case some small differences, usually down to cost-cutting, can be found.   Not every Gamages' lathe was by Portass, the company being well known for re-branding lathes from several other makers as well. If you have an example of a Portass badged as other than the examples already discovered (those with hyperlinks) the writer would be interested to hear from you.