If you do a Google search for WHAT'S MY BSA then one of the first hits that shows will take you to the list of prefix letters on the John Knibbs website.
It's quite a long list, with all the variations used on the different models, including for mark, year and calibre.
The website has always had this available, but it was easier to find it on their older style site rather than the current new one.

Best regards, Clogger