All the Triumph factory records were lost in the Coventry blitz but no other factory seems to have suffered to the same degree. It's not clear why the surviving BSA factory records suddenly stop, but they're more complete than Norton who only have most of 1939 and one book from 1940. The most likely explanation is that the records served initially for guarantee purposes and then later for spares and technical enquiries. BSA had to be able to supply the correct parts to a dealer who quoted pre-war engine and frame numbers...but at some point it was realised that 20lb book after 20lb book listing machines with no differences and only one customer - "War Office, London" simply weren't worth the shelf space.
The fact that your bike is not in the Australian Army records and has a correct year of manufacture would suggest to me, certainly in a British context that it was probably delivered to a government agency who used civilian rather than military serials and registered immediately. Maybe fire brigades, coastguards, that sort of thing.
