From what little evidence survives, it appears that the few lathes advertised by Tom Senior were not constructed in his works, but by Arthur Firth Ltd., the Company where he served his apprentice. Firth were long-established in the area and advertised widely in the engineering press selling machines under their own name and also branded as "Atlas" and "JSB".  Senior must have had a particularly well-developed mechanical aptitude for, by the age of 19, in 1919 he had risen to the position of foreman Although this is pure conjecture, he must have found such a position of responsibility difficult to handle, not through any lack of practical knowledge or skill, but because of what must have been many awkward situations when he had to deal with men far older and more experienced than himself - "T'young bugger, he knows nowt…". So, with just £3 in savings, and not yet at his majority (at the time 21 years), he decided that the time was right to branch out on his own. Fortunately, he had excellent relations with his previous employers and would, as was the custom, have undertaken a good deal of (not very profitable) sub-contact work on their behalf. As Senior's business grew he began to take on the role of tool and machine-tool distribution, handling the Firth lathes and hand planers - long a staple of the company according to contemporary advertisements - and possibly buying in casting made from Firth-owned patterns as demand dictated. By 1925 Senior had expanded sufficiently to take over the Firth business with an advertisement in the Model Engineer and Light Machinery Review announcing that "Atlas" products would now be made by Senior.
Lathes from Arthur Frith were of distinctly old-fashioned appearance, even for the early 1930s. The first machine produced is believed to have been a 3.5" centre height by 15.5" or optionally 24" between centres machine that was born into a competitive and hard-up world and, is it suspected, very few were manufactured. Described as the "Yankee" in some catalogues, the most common specification was a  9/16"-bore, No. 1 Morse taper headstock spindle that ran in plain bronze bearing. While the backgeared headstock appears to have been heavily built and capable of handling proper jobs, the flat-topped, V-edged bed, with its strangely-positioned feet, was noticeably shallow in depth. Unusually for a lathe of this size the tumble reverse was contained on the inside of the casting, where it was better supported and protected. A large set of 16 DP changewheels was provided as standard: 20t, 25t, 30t, 35t, 40t x 2, 45t, 50t, 55t, 60t, 65t, 70t, 75t, 80t, 85t, 90t, 95t, 100t, 105t, 110t, 105t and 120t. They drove to a 3/4" diameter leadscrew of determinedly Victorian specification with pitch of 4 t.p.i. and a square section thread.
Useful T-slots ran across the full width of the saddle both to the front and rear of the cross slide but the carriage traverse handwheel, which drove through reduction gearing on to the bed-mounted rack, was the of balanced type, fitted awkwardly on the end of a long (and old-fashioned looking) stem. Although the swivelling top slide was graduated with degree marks, neither of the compound-slide feed screws carried a micrometer dial.
With a square-thread spindle that ran clear though the handwheel, and locked with a proper compression fitting, the tailstock was more up to date and of a design seen on lathes made into the 1970s.
Like many makers, Firth offered their lathes both for bench and stand mounting, in the latter case with a 35" x 8" cast-iron tray (complete with either wall or ceiling-mounted countershafts) or equipped with a self-contained treadle drive that turned a 72 lb flywheel with steps of 191/2", 183/4" and 18" for round "gut" rope to drive a headstock pulley with steps of 215/16", 311/16" and 47/16". Amusingly, because the feet were cast as one with the bed, the bench lathe was simply bolted to a low cast-iron stand when ordered with treadle drive. A long-bed version of the lathe with increased centre height fared little better in the appearance stakes: with its bed feet left in the same position as the smaller model, the bed at the headstock end was cantilevered out to a ridiculous extent.
If you have a "Firth", "Senior", "Atlas", "JSB" or Senior lathe, the writer would be very interested to hear from you..

Arthur Firth 3.5 x 15.5" lathe of the 1930s as marketed by Tom Senior. An instant identification point is the enormous boss on the leadscrew clasp-nut operating handle.

A long-bed version of the Firth lathe with increased centre height. Note the enormous overhang of the bed from its headstock-end foot.