Service records don’t exist for motorcycles, and the factory ledgers for this contract (C/13290) do not exist (anymore?). But we can give you some general information about your bike:
Contract C/13290 was a contract for 10.000 bikes (5000 RASC, 5000 RAOC). Frame numbers were #71818 - #81817. There were two blocks of census numbers: C4752401 - C4757400 and C4860801 - C4865800. Your bike is the 495th from this contract, the British census number would have been C4752895. Production of this contract started at the end of November 1942, @ 400 bikes per week (approximately). This means that your bike must have left the factory in December 1942. (I've added a picture of a "sister bike", which must have rolled off the production line one day before your bike).
WM20.72312 was then transferred to the Australian Forces, where its was given a new (Australian) census number 90068. This number can indeed be found in the AWM ledgers, without further information.
email (option): wd.register@gmail.com
This is another one that is a close cousin with the Australian number on the tank - only 26 different in frame number.
This photo was taken in the Northern Territory and is the only period photo I have seen with what looks like it may have an Australian Bryant air filter fitted.
Ray
email (option): rays54@hotmail.com