Hi Roger...Only in some years..It depends largely on the frame type, or more particularly whether more than one frame type was produced in the year in question...
Plunger models had an 'S' suffix after the frame number to indicate 'sprung frame' (but not on the engine) and rigid and plunger models in the same year ran different number sequences...
All B33 engines were fitted to frames with B31 numbers...This was normal post war practice for BSA in the 40s 50s and early 60s where the frame numbers for the smaller model were used...
One interesting exception, though not relevant to your question, was the Rocket Gold Star...These had A10 prefixes on the frame, not A7....
If you can provide the year and model (frame type) of the bike you are talking about I could be more specific...Ian
email (option): ian@wright52.plus.com
Hi Roger...For 1952 B31 plunger models, engine numbers commence at ZB31-21001 and frame numbers at ZB31S-17001...
So both numbers should be above those numbers and they would run sequentially from the beginning, although they were not actually matching...
1952 was the last year of the 'ZB' prefix to frames and engines..
From 1953 they began with 'BB'...Ian 
email (option): ian@wright52.plus.com