The Drummond B-type isn't usually 10tpi, it is usually 8tpi
The Drummond Roundbed is 10 tpi.

The tables for the 10tpi are on the Groups.io Drummond group files section. Somewhere. However it does depend on what set of changewheels you have. The early ones had even numbered wheels while the later ones have steps of 5.

I also published some combinations for metric threads for both 8 and 10 tpi on the groups site in a thread not the files some 6-9 months ago.

Changewheels for many lathes often increment by 5T, so at a wild guess, you could be looking for 35, 40, 45, 50, 55T.

An extra 20T or two may be useful, as possibly another 60T.

Arranged to give large enough ratio between chuck and Leeedscrew, you can get quite a low feed rate for turning. (You would be looking for something of the order of about 27 to 30:1 with a 8 tpi Leadscrew ).

The 63T should be useful if you want to cut Metric threads. It won't be absolutely accurate, but as Brian Wood's book shows, the errors can be small enough to be acceptable.

The Back Gear will allow the spindle to driven at a lower speed, (Possibly 6:1 slower? ) which can be very useful when screw cutting.

But don't forget to unlock the bull gear when using back gear, or everything will lock solid, at best; or do damage at worst.

Howard

Chris the "official" list of gears for your lathe would be 2×20, 2×30, plus 1 off of each of the following 35, 38, 40, 45, 50, 55, 60, 65 and 1 off 63 to allow for "near enough" metric threads.

Keith

Good use of 3D printing there, I like it.