Gearbox Identification

[mickm]

I run a 1957 single. At the moment it has a Burmah box. I want to return it to an amc box.These are around but I am puzzled by the markings on some of them. I think I know the difference between AMC and Norton castings. The AMC box has it's lugs at 12 and 6 O'clock, whilst the Norton is 12 and 4 O'clock. My question is that whilst our boxes have an ID No starting with the letter M, they sometimes finish with the letter G. can someone please explain to me what this all means? once I have finished the mods I'll have a Burmab box with a new clutch for sale. Unless that is someone would be interested in an exchange? Mick from North Essex.