It could be 38th (Welsh) Division, the cap badge looks like Royal Welch Fusiliers, formed summer 1939 as a second line Territorial Army Division and was part of Home Forces, A yellow cross of St David on a black shield on a khaki background, although the background may have only been on the cloth badge?
Yes it is hard to see but on second look it could indeed be a shield instead of a square.
I was asking because I have this tank with a red square in about the same position:
Also the C number is very close to the one on this tank - C4344630 - so I thought it had to be the same sign, but probably not, the background of the cross also looks too dark to be red....
With most motorcycle makes, the C numbers for each contract are calculated in order of frame numbers. I have learnt however that Velocette didn't bother to follow this procedure to the letter!
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.