It took a little over 10 years for a screwcutting gearbox to become available as an option for the Atlas lathe but when it was introduced (in 1947) it was also offered as complete kit to enable the conversion of earlier lathes--with either the 3/4" or 5/8" diameter leadscrews. An unusual Pick-o-Matic semi-gearbox was also available, where gears, clustered into sets , were supplied pre-arranged to speed up the thread-cutting process.
Unfortunately the special handbook issued with the gearbox omitted several key factors in its explanation of how to generate metric pitches--and this was not corrected until a new edition published for the same gearbox when fitted to the later (1959) heavily revised 12-inch Atlas. However, there is a simple way (cleverly worked out by Atlas-owner Carvel Webb) to generate a limited range of the more common metric pitches. Because the 30 t.p.i.,  24 t.p.i and 20 t.p.i positions represented the same ratio intervals as 1 mm, 1.25 mm and 1.5 mm pitch if one could convert the 30 t.p.I. position to 1 mm pitch, then the 24 t.p.i and 20 t.p.i positions would correspond to 1.25m and 1.5mm. Now, 30 t.p.i = 25.4/30 = 0.8467 mm. Close, but not close enough, so how to get this closer to 1 mm ? Using 52 and 44-tooth gears gives 0.8467 x 52/44 = 1.0006 mm - which is almost spot-on. The tumbler gears in the Atlas set include a 32/16 shielded gear driving a 40 tooth sliding gear which in turn drives the box through a back-to-back pair of 48-tooth idlers. Because the 52/44 combination fits neatly in place of the 48/48 idler the job can be done. With the 40-tooth sliding gear engaging the 52-tooth gear to the box, the 52-tooth gear is acting as an idler (with the 44-tooth gear as its spacer) and the gearbox operates in its normal `Imperial Mode' - but with the exception of the coarsest (seldom-used) range. However, with the 40-tooth sliding gear in the other position, it engages the 44-tooth gear of the 52/44 combination and introduces the 52/44 ratio into the train. The 30, 24 and 20 positions, or 60, 48  and 40 positions, become the desired 1 mm, 1.25 mm and 1.5 mm pitches, depending upon whether the 52-tooth gear of the 52/44 pair has been placed closest to, or away from, the headstock. However, placing it away from the headstock maintains the bulk of the t.p.i. and feed settings (as per the chart on the gearbox). It also means that it's not necessary to remember to double everything - as would be the case if the sliding gear was driving off the 32-tooth part of the shielded tumbler gear. All that is now required to switch between the normal "Imperial Mode" and the "Basic Metric Mode" is the few seconds it takes to loosen the yoke and to move the sliding gear in or out.