BSA WM20.96799 would indeed have had the census number C5130199 on its tank. It is one of the last bikes from contract S/1048 (15.000 bikes, census numbers C5115218 - C5130217). Production of contract S/1048 started in June 1943, at that time BSA produced approximately 2.000 bikes per month. Which would mean that WM20.96799 must have left the factory in early 1944.
These are pictures of two BSAs with "close" census numbers. Both pictures taken during the liberation.