Australian Hercus "Model 260" lathes used a slightly different arrangement and the conversion gears for those are:
Hercus Model 260-AM lathe with a 3 mm pitch leadscrew lathe and a (metric) screwcutting gearbox had the following gears fitted as standard:
18, 45, 60 & 80 idler. To cut a full range of inch-pitch threads you would need the following additional gears: 25, 33, 39, 42, 48, 50 and a 63/64 combination gear.
need the following gears to generate a full range of metric pitches:
20, 24, 25, 30, 33, 40, 42, 50, 80 and a 60/63 combination gear.
Hercus Model 260 BM and 260-CM lathes with a 3 mm pitch leadscrew and changewheels for screwcutting need the following gear set to generate a full range of inch pitches:
18, 24, 25, 30, 33, 36, 39, 42, 48, 54, 60, 72, 80 and a 63/64 combination gear
Hercus Model 260-B and 260-C lathes with an 8 t.p.i leadscrew and changewheels for screwcutting need the following gear set to generate a full range of metric pitches:
18, 20, 24, 27, 33, 36, 40, 42, 48, 54, 60, 63, 72, 80 and a 60/63 combination gear.