Gord,
The gearbox shell is identical to that used on the contemporary TT model as well as the later DT/SW models. What makes the unique are the serial number's prefix, and the internal gear ratios they denoted. For the OC, three options were available; UG (presumably normal, and also the TT model), TG (touring, wide ratio), and SG (close ratio). There is also VG, very wide ratio as used on the TT; and YG, which was the ultra close ratio gearbox used on the DT. Later SW models used the UG or TG.
So shells abound, the problem is the gears. The most abundant gearbox one comes across is the YG. You may start to slather thinking about sporty pretensions of fitting an ultra close ratio 'racing' gearbox. Forget it! The interbox ratios are 1:1.5, 1:1.25, and 1:1 (direct.) There is not enough spread to make a useful road going three speed gearbox. Not unless you like wringing the engine at 55mph maximum, or alternately having a first gear that starts at 25mph. These boxes were intended to be stuffed in one gear that best suited the length of the track, and left there.
Gearsets for the road ratio boxes are much, much more scarce. This is the problem I ran into when I rebuilt the transmission for my SW, and I ended up making new gears. I did manufacture some spare sets, but they sold out long ago. There are 2nd gear pairs still available. This is a link to the old advert, if you manage to find a gearbox.
https://www.douglasmotorcycles.net/index.php?topic=117.msg276#msg276I need to do a little research on the hub situation. They might be common to other models of the era.
Doug