Help!!! Ive been told that this frame is a 1965 Thunderbolt Rocket but the "A50" denotes a 500cc engine while Thunderbolt was a 650cc. Any ideas?
I think all the frames were marked A50 the same as all A10 frames were marked A7.
Idie is right (up to a point) .... Most A50 and A65 had A50 (some with additional letters - like A50A / A50B / etc.) until the factory used matching engine and frame numbers (about 1966 / 67), when they then had the model number included on the frame (e.g. A65L, A50RA etc.).
There is a list of post 1960 engine / frame number ranges on the website ... https://www.bsaownersclub.co.uk/number-display/6