I'd start in the obvious places. The VMCC Library holds a 1929 catalogue, an illustrated spare parts list, etc, etc as does BMS publications who seem to hold even more. They are both on line.
This is the joy of owning a popular make like Triumph. You don't even have to move away from your computer to order all the info printed by the factory. Wish it was that easy with some of the other makes.
Best of luck, it looks like a nice bike you have there.
Cheers,