The BSA with frame number (and also duplicated frame number on the engine) WM20.59208 was despatched from the Birmingham factory on 30/4/42, destination Ministry of Supply, London.
It was part of contract C/11101. This was a contract for 10.000 bikes (frame numbers 53413 - 61412 and 61448 - 63447). The census number on the tank would have been C4662951. As Ron said, the brass contract plate on the rear mudguard refers to this contract. Here’s a sister bike:
During the war this bike was transferred from the British Army to the Australian Forces, as I found frame number WM20.59208 in the AWM ledgers (I guess you’re in Australia Gary?). In Australia the British census number was replaced with AMF number 39026. After the war it was sold to Finlay Bros.
The BSA with frame number (and also duplicated frame number on the engine) WM20.35572 was made under contract C/7287. This was a contract for 17.000 bikes (frame numbers WM20.25001 - WM20.42000). Unfortunately the ledger page containing this number is lost. But from the previous and the next pages we can see that it must have left the factory in early April 1941. The original census number would have been C4352164.