Welcome!
Lots of folk around, some of whom may well be familiar with your lathe, and be able to give useful advice.
Changewheels, sometimes in addition to a Norton gearbox, are used for screwcutting. Changing the gear train, alters the ratio between the chuck and the Leadscrew, to alter the pitch of the thread.
On my particular lathe, depending on the combination, threads from 4 tpi to 112 tpi, or metric threads from 6 to 0.25 mm pitch can be cut (If I was prepared to produce some real non standard oddballs, even finer pitches could be cut )
Often, the leadscrew is used to provide a power feed, so that a suitable train can produce a fine feed with the objective of getting a consistent fine finish. The finest that mine will produce is 0.0047"/ rev or 0.12 mm/rev depending on the set up.
Gears from one make of lathe may not suit another marque. (Gears for early Myford lathes (ML1,2,3 and 4 ) are dimensionally the same as for the later 7 Series, but the early drive via pins, where the 7 Series use keys.
Not only could dimensions differ,but also the Diametric Pitch, and /or the pressure angle.
So check that any gears that you obtain are compatible.
Often, a complete set would consist of gears with tooth counts going from 20 upto 60, or more, incrementing by 5, with some duplicated. (A mini lathe will have 2 x 20T and 2 x 80T gears, for instance. )
HTH
Howard
Howard