The brass plate for contract C11101, relates to your engine only 59208 (1942) Your frame 91162 is from contract S1048 (1943) and would have had tank census number C5124562.
Here's a bike from that contract. Ron
email (option): ronpier@talk21.com
I'll try to add some information to what Ron already mentioned:
The bike with frame number WM20.91162 is a contract S/1048 bike. This was a contract for 15.000 bikes (frame numbers 81818 - 96817). The factory ledgers for this contract didn’t survive, but we know from the receipt cards that the production started in early June 1943, at 2.000 bikes per month. This means that your frame must have left the Birmingham factory in October 1943. The original census number would have been C5124562. Here’s a very close number:
email (option): wd.register@gmail.com
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.
email (option): wd.register@gmail.com
Here’s a picture with some Australian sister bikes:
email (option): wd.register@gmail.com
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.
Here’s a sister bike:
The bikes with ZM numbers are post war.
email (option): wd.register@gmail.com