The bike with frame number WM20.6880 has left the Birmingham factory on 10/10/1939, as part of military contract C/5110. This was a contract for 8.100 bikes (frame numbers WM20.5001 - WM20.13100).
The original census number would have been C3930300. Here's a line up with close numbers:
I'm not sure about the engine number though...
The bike with frame number WM20.34492 has left the Birmingham factory on 14/03/1941, as part of military contract C/7287. This was a contract for 17.000 bikes (frame numbers WM20.25001 - WM20.42000).
The original census number would have been C4351084. Here's a sister bike:
This number appears in the KeyCards, which means that it was used in the post war British Army. The post war registration number was 06YD14. It was indeed demobbed in 1967. A hi-res scan of this KeyCard (#129a) can be obtained from Lex. Here's a post war picture with a close number:
The engine with duplicated frame number WM20.29740 also comes from contract C/7287. The complete bike was despatched from the factory on 13/11/1940, and would have had census number C4346332. The frame of this bike is also in the KeyCards (#170a), but the duplicated frame numbers on the engines were never recorded in the KeyCards.
Official dating certificates can be obtained from the VMCC.
Hope this helps... 😊
email (option): wd.register@gmail.com