The original AJS company, way, way, back before Matchless bought them in 1931 and way before AMC was ever formed identified each years production with a letter prefix. By 1930 they had reached the letter 'R' so both the frame and engine numbers of all 1930 AJS commenced with the letter 'R'. That year they made the models R2, R4, R5, R6, R8, R9, and R12. I know this because I am copying it directly out of my 1930 Export Catalogue.
Not only that but AJS were one of the very few companies who really did have matching numbers for the frame and engine. If the bike is absolutely original the number on the engine should be exactly that as on the frame.
So, if you were to provide a little more information we might be able to help a little more.
Cheers,