Hi. The frame is 1952, plunger suspension type, but BSA started numbering the frames from autumn of the previous year. Yours is fairly early in the sequence so may have left the factory very late 1951 or early '52. The engine number you have provided is incomplete. There would normally be two letters preceding the numbers, AA7S 2101 would be 1952 and likely to be the engine which was fitted to your frame when the bike left the factory.