Smart & Brown Model-M 4" x 18" This lathe was based closely on the pre-war German Boley L3/L4 with its 6 t.p.i. leadscrew disposed centrally under the bed and the whole of the changewheel bracket arranged to swing in order to provide a "tumble-reverse" to the leadscrew drive. However, the design was not left entirely alone, S & B deciding to fit a conventional full-width backgear assembly instead of the original type that was clustered up against the front spindle bearing - though they did retain the helical-form gears. On the Model M Mk. 2 Smart & Brown, further modifications were made including a neatly-engineered power shaft along the front of the bed to drive the sliding and surfacing feeds. On some versions of the Mk. 1 (including one that passed through the hands of the writer) the cabinet stand was in cast aluminium, through most seem to have been in cast iron. The base contained a motor, mounted on a simple adjustable plate fitted to slide rails that allowed the easy fitting of alternative makes and sizes. A typical, original fitting would have been a 3-phase, 3/4 h.p. Thomson-Houston 940 r.p.m. unit equipped with a 3-speed flat-belt pulley driving direct to the spindle to give nominal speeds of 411, 787 and 1432 r.p.m in open drive and, through the 5 : 1 ratio backgear, of approximately 82, 157 & 286 r.p.m. Some models are believed to have been fitted with a 3-speed gearbox in the cabinet base, in which case the number of speeds totalled 12, through with a very similar span from slowest to fastest.
For its size, the lathe had a generously large spindle thread - 1 ¾" x 8 t.p.i. backed by a long (1 ¾" ) register length behind. The spindle was machined to take direct-fitting collets with a 25 mm diameter body - though if it had not been for this arrangement the bore and thread could easily have been made larger still. Interestingly, the collets are almost identical to the Schaublin Type W25 - though the seat in the headstock spindle differs by a couple microns - probably a case of S & B putting the spindle bore "on size" whereas Schaublin, instead, put the collet on size" - and added 5 microns to the spindle bore.
Built in Mk 1 and Mk. 2 versions, the first Model-M appeared during late 1946 and the latter - considerably modified - in 1948. Production ran, it is believed, until 1959. More details of the lathe's background and construction can be found on the Boley page..

A just post World War 2 Smart & Brown Model M Mk. 1 on a cast-iron stand and looking very much like the Boley lathe from which it was descended. This model is now  rare.

A very early example of the Smart & Brown Model M missing  its louvered covers on the face of the stand. The top slide on this lathe has been modified

For its size the lathe had a generously large spindle thread - 1 ¾" x 8 t.p.i. backed by a long (1 ¾" ) register length behind. The spindle was machined to take direct-fitting collets with a 25 mm diameter body - though if it were not for this arrangement the thread and bore could easily have been made larger.   

As on the Boley original the whole of the changewheel bracket was arranged to swing in order to provide a "tumble-reverse" to the leadscrew drive