Here a copy of a page of the original BSA ledgers, this pages shows that your bike had matching engine and frame numbers and it left the factory 23-6-1941 and it went to the War Office in London.
Out of interest, are there any estimated figures as to how many M20's stayed in the UK during the war years,which production batches they tended to be or were not used at all, held in reserve etc. or how many were returned to the UK from europe etc. Just curious..