Time for a small update on the (presumably) Australian broad arrow markings. Thanks to the forum members we've discovered some more examples, and thanks to Rob Miller I've been able to check if the respective BSA engine numbers are indeed listed in the AWM ledgers. Here's what I found:
- WM20.35502 (broad arrow stamped on rear frame): not listed in the AWM ledgers
- WM20.41037 (broad arrow with part of a serifed letter M stamp on engine): listed in the AWM ledgers (AMF number 13292)
- WM20.46289 (broad arrow stamp on engine): listed in the AWM ledgers (AMF number 30538)
- WM20.49952 (broad arrow stamp on engine): listed in the AWM ledgers (AMF number 37543)
- WM20.54713 (broad arrow stamp on engine): listed in the AWM ledgers (AMF number 38890)
- WM20.57460 (broad arrow stamp on engine): listed in the AWM ledgers (AMF number 83515)
- WM20.78362 (broad arrow over a letter M over 397 stamp on engine; this is a British Army acceptance marking): not listed in the AWM ledgers

Conclusion so far:
We know that the "broad arrow over a letter M over a number" markings on the WD motorcycle engines (or on the Welbike frames) is a British Army "acceptance marking". See also this thread.
I think it is now reasonable to assume that the "lonely" broad arrow stamp on an engine was typically used by the Australian Army. And it looks as if they also had a "broad arrow over (part of) a serifed letter M" stamp in use.

More examples from down under are of course always welcome! If you don't know how to post pictures here, please send them to me in an email, and I'll post them here for you.

Thanks for your help in resolving another mysterious marking!
Jan

email (option): wd.register@gmail.com