The bike frame is 20 S 5341 which is a 1955 BSA M33 Plunger and the engine is a M33 OHV engine, but at some point its clearly had engine work. Perhaps a new engine or perhaps a new case... who knows.
C20 2 2102 5
The Middle 2 is much bigger than the rest and the 5 is about 1/2inch to the right of the 2102.
Any ideas around those numbers?
Hamish