Hi Clive,
I think what you probably have is an A10 basket case from 1957 and a 1949 A7 engine. The plunger A7 was discontinued in 1954 but the A10 plunger was made until 1957. BSA generally used the same frame numbers for the A7 and the A10 so, with the exception of the A10 Super Flash, all A7 and A10 frame numbers started with a prefix letter and then "A7".
Perhaps you might consider looking for a 1957 plunger A10 engine instead of a 1949 A7 frame. You could then have a Plunger A10 with a spare A7 engine.......just a thought!!
Beezageezauk.