Comment
I don’t have any post war records (XM20) but I may be able to help you with the WM20 engine: WM20.62399 rolled off the production line on June 23th 1942, as part of contract C/11101. This bike would have had census number C4666107 on the petrol tank. Here's a picture of C4666503 (WM20.62795), which was despatched from the factory on June 24th 1942, one day after your bike.
email (option): wd.register@gmail.com