Both the Model C and Model B 9-inch lathes used changewheels for screwcutting whilst the Model A was always fitted with a screwcutting gearbox.
Although all but the very first lathes (the Model 504) were fitted with a tumble-reverse mechanism it was not until the late 1950s that South Bend include a spring-loaded tumble reverse selector lever in the specification. Until then, to reverse the direction of the carriage movement, or to disengage the drive, a square-headed spanner (wrench) had to be used to slacken the pivot arm before it could be moved. The English copy by Boxford employed a much more convenient and quicker acting spring-loaded mechanism from its first production model in April 1948.
Although the vast majority of 9-inch Workshop lathes for the American market were supplied with 8 t.p.i leadscrews numbers were manufactured for export with a 6 mm pitch metric leadscrews in both Model B and C form (with changewheels) and as Model As with proper, all-metric screwcutting gearboxes. The English and metric screwcutting gearboxes were different externally, and very easily recognised the former having the diagonal line of indent holes on the right hand half of the box's front face, the latter with them across the left-hand half.   
"English" threading Lathes with an 8-t.p.i leadscrew and changewheels for screwcutting (i.e. without a screwcutting Gearbox) were supplied with the following changewheels when they left the factory: *16, 24, 36, 40, 44, 46, 48, 52, 54, 56, 60, *80,  *72/18 compound,  *80 idler (with boss)
(* gears on machine as dispatched from factory for standard feeds)
To convert a Model B or C (non-gearbox) English-specification lathe to cut metric threads the following gears are needed: 20, 100, 127/100 combination
In addition, to cut the following five pitches extra gears are required as follows: 0.45 mm = 18t gear,   0.55 mm = 22 t gear,   0.65 mm = 26 t gear,  0.7 mm and 3.5 mm = 28 t gear. 
Metric Threading Lathes with a 3mm pitch leadscrew and changewheels for screwcutting (i.e. without a screwcutting gearbox): were supplied with the following gears as standard. *16, 24, 28, 30, 36, 40, 44, 48, 52, 56, 60,  *80,  *72/18 compound,  *54/18 compound,  *80 idler (with boss)
(* gears on machine as dispatched from factory for standard feeds)
To convert a non-gearbox Metric-leadscrew lathe to cut English threads the following gears are needed: 18, 22, 26, 38, 54, 64, 88, 135/127 compound, 48/24 compound.
"English" threading Model A lathes with an 8 threads-per-inch leadscrew and a screwcutting gearbox had a standard ex-factory drive train consisting of: 20t,  40t, 56t and an 80t idler. To convert this gearbox to cut Metric threads the following gears are needed: 24, 26, 28, 32, 36, 44, 48, 127/100 compound
Metric threading Model A lathes with a 3 mm pitch leadscrew and a screwcutting gearbox had a standard ex-factory drive train consisting of: 20t,  45t,  50t and an 80t idler. To convert this gearbox to cut English threads the following gears are needed: 38, 40, 44, 52, 56, and a 135/127 compound
As an alternative to the 127/100 gear for metric conversion with a standard changewheel lathe it is possible to use a 37/47 transposing gear and then use a formula to calculate the necessary gear trains, as follows:
Pitch of thread in mm = 2.5 x (stud gear teeth divided by leadscrew gear teeth) or, to find the gear necessary to fit on the leadscrew: leadscrew gear = (2.5 x Stud Gear) divided by Pitch in mm
All the gears necessary to generate metric and other pitches are now available at a good saving on the factory price.
The gear on the end of the spindle - and the two gear tumble-reverse gears beneath that match with it - are 20 DP. The tumble-reverse output gear, and the train of changewheels it drives, are 18 DP and the gears inside the gearbox 16 DP.