BSA's first wartime contract was all over the place, but the system did improve and from contract S1048 onwards, serials commencing C5115218, Frames WM20 81818, the two last digits corresponded. This would have been around May 1943.
Michiel, where did your tank come from ? I assume that it starts C4344*** ? I had hoped that we could reduce the choice to BEF formation signs but this bike must have been delivered later than August 1940. If it was in German hands, then Most likely ex-Greece. I've not heard of them bringing vehicles back from North Africa.