This is Mauro's picture:




The fact that there is no "engine number" (we are actually referring to the duplicated frame number, as the frame number is master, and the so called engine number is just a copy of the frame number) probably means that this was a spare engine (so it wasn't fitted to a frame yet, hence no duplicated frame number)

The BB marking is still defeating us. Some engines have no B marking at all, some have a single B, some have a double B. We haven't found any logic yet. It could be a BSA inspector's stamp...?

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

The quite deep BB stamping reminds me of the HC stamping in some OHV engines - high compression.

Maybe it refers to a certain specification of the engine.., Ball Bearing ??? :thinking_face:

Interesting ... thanks for this information and assumptions ...
There is still quite mystery behind these motorcycles