Gord,
The OC, TT, and DT/SW shared the same gearbox shell. It was different from the previous OB/OW gearbox in that the position of the main and lay shaft were swapped around. The main shaft was now at the forward end of the gearbox. Later on, the DT gearbox was reinforced and changed shape slightly, and there was a variation used on the F/G31 that had provision for a kick starter.
The only difference between a OC, TT, or DT/SW gearbox are the internal ratios, and what prefix code is stamped into the serial number. The OC, TT, and SW used a "UG" prefix box, which had conventional road ratios. The "YG" prefix is the most commonly seen, and use on the DT; it has ultra close ratios. You might think this would be desirable for your rebuild, but you would be wrong! The ratios are so close, that you cannot get a decent spread between low and top gear. Due to lack of choice, folk have used them in DTs converted into SW spec for road use but they are not satisfactory. You either set the bike up with sprockets to give a normal first gear and the engine is screaming its guts out at fifty mph, or you set it up for a decent cruising speed with a first gear good for 25 mph before the clutch is fully home! The "YG" gearset was really intended for Dirt Track racing where you did not shift gears during the race. You selected one gear that best suited the size of the track (1/8, 1/4, or 1/2 mile ovals) and you left it there for the duration.
There was also:
"TG" prefix, wide ratio used for the OC model.
'SG" prefix, close ratio used for the OC model.
"VG" extra-wide ration used for the TT model.
Gearbox shells are not impossible to find, it might take a while but they are about. Decent gears on the other hand are in short supply (hence more shells around than gears!) My SW5 had DT ratios inside the "UG" prefix 'box, so I made a few sets of the road ratio gears. The full spare sets sold long ago, only 2nd gear pairs remain.
If you go the Norton gearbox transplant route, it is important to build keys into the gearbox joint face to mate with the shallow grooves cut into the frame lug. The chain exerts quite a powerful pull on the gearbox, trying to wrench it free from its moorings. Early conversions that just relied on the studs failed as it just hammered the studs back and forth until they loosened up in their holes.
One of my DTs came from Ontario, where it was used for hill climbs in the forties by Richard Gower.
-Doug