By the late 1930s South Bend had two 9-inch lathes in production: the lighter 9-inch "Workshop" model introduced in 1934 as the Model 504 and the very much heavier Type R, a lathe also built as an 11-inch model in several versions and whose origins went back to the early 1920s. During 1939, in order to put some distance between the two 9-inch models, the "R" was revised becoming what is believed to have been the Model T (with a 9-inch swing) and then (looking almost identical) as the 10-inch swing "Heavy Ten", a machine that, due to its compactness and useful features, was to prove enormously popular and remain in production (though in a modified form) until the late 1990s (a well-preserved version from 1967 can be seen here)
Thousands of Heavy 10s were exported world-wide and many are still found in UK workshops doing "sterling" service; another version of the lathe, always found with a rear-drive countershaft, was also marketed in Great Britain as the UNITOL. These (and some versions of the Model R) were completely stripped of all South Bend badges, even to chiselling off letters cast into the bed. For the UK a screwcutting gearbox and power cross feed were part of the normal catalogue specification but for the USA, and other markets, the lathe could be had with changewheels for screwcutting and even as a rare "Junior" model with a plain apron and hence no power cross feed - and longitudinal feed by setting the changewheels to a fine-feed compound reduction. Strangely, for a lathe intended for machine-shop and general workshop use, the original screwcutting gearbox had a single tumbler and its threading range was, accordingly, limited; however, in 1949 a much-improved twin-tumbler box was introduced and the threading range greatly expanded. The lathe was available with several different drive systems: for bench mounting with a rear-mounted self-contained motor and countershaft drive; on plain cast-iron legs for drive from a built-on countershaft and motor or equipped for drive from overhead line-shaft or countershaft system. The lathe could also be supplied as a ready-to-run "underdrive" model (in which form it appears to be most common) and fitted to one of two very different underdrive cabinets stands. The more modern of these was a sheet-metal encased affair made from steel tubing with a large chip tray and useful storage drawers whilst the other, with a heavy cast-iron plinth under the headstock and a separate leg under the tailstock end, looked rather old-fashioned even for the late 1930s. The countershafts used in the underdrive models followed the design of that for the 9-inch where a V-belt from the motor passed over a narrow flat pulley on the countershaft, a system confusing to modern eyes but one that works well in practice. Final drive was by flat belt with an enormously powerful over-centre mechanism to tension the belt. Whilst most Heavy 10s were fitted with the extra-cost double-step pulley on motor and countershaft (which gave a high-speed range and doubled the number available) occasionally models are found with the far-less-used single pulley arrangement. Speeds with the standard pulleys ran  through: 50, 79, 129, 277, 434 and 70 r.p.m. and with the high-speed set additional speeds of 97, 153, 248, 535, 837 and 1357 r.p.m. (though these particular figures did differ over the years and were also altered by the use of 1700 r.p.m. motors in the USA and 1425 r.p.m units in the UK).
Manufactured to South Bend's normal high standards, the headstock spindle was hardened and "superfinished" to better than 0.000005" r.m.s. Lubricated by a filtered, circulating oil supply it ran in either bronze or cast-iron bearings with their upper and lower halves separated by special "peel-off" laminated shims that could be arranged to give the correct clearance - these shims are still available (picture below) and can be had from surfwerks@gmail.com An interesting point concerns the spindle nose: this used what appears to be a "short" No. 5 Morse taper - but was actually specified as a proprietary fitting with the same taper rate as a No. 3 Morse (0.602" per foot) but with a "gauge line" of 1.629" diameter. The "proper" No. 5 taper has a rate of 0.6315" per foot, and a gauge line of 1.748" diameter. Although calculations show that a standard (but shortened) No. 5 Morse taper 2.5-inches long would be approximately 0.006" loose at the smaller-diameter end in practice a No. 5 Morse does appear to fit - though there is always the chance that your lathe may be different. The correct sleeve to South Bend's original specification (and it's worth having one) can be obtained from Scot Logan at lathe.com
As was the case with many South Bend models from the early 1930s onwards, both "standard" and "toolroom" versions were offered; there was no difference in production tolerances between the two only a difference in specification with the better equipped toolroom version having (in most years) a taper-turning unit, draw-in collet attachment, collet rack for attachment to the tailstock end of the bed (but no collets), thread-dial indicator, thread-cutting stop, large faceplate a micrometer stop and, the one concession to a genuine toolroom specification, a leadscrew cut to an improved (but undisclosed) degree of precision.
With the ever-increasing need to generate metric pitches it is worth knowing that the metric conversion set for the Heavy 10 included a special forked changewheel bracket to replace the standard single-slot affair. Whilst it is possible to generate metric pitches with the latter in place (and using the correct changewheels) the range is somewhat limited..