gearbox stamps

[a10col]

Hi to all this is my 1st post
I am building a a10 plunger 1952, I bought  the bike in a loosely built condition with three gearboxs and loads of bits missing the problem is i have no idea what gearbox should be fitted out of the three or even if any are the correct one.
I have seen a few on ebay and the stamp thay have is STD, but the boxs i have only have a single letter on each in the same place, box 1 has the letter R,
box 2 has the letter P, and box 3 has the letter L,. Can anyone help me identify what i have ?
Thanks in advance for any help
Colin.

[muskrat]

 G'day Colin, welcome to the forum.
                                                Are your letters stamped in the same spot as the std at the top of the inner cover? Your inner cover might have been a replacement part and not stamped as usual, but with the fitters inital. Throw up a few pics of your bits.
Cheers