Yes you are right about these painted crank cases. Here an image from the booklet about these India Office BSA's, cleary unpainted aluminium.
Henk
There is the way BSA supplied them, and the way the army used them
Which is correct ?
You all argue till you are blue in the face about regimential markings when no BSA had them on when they left the factory .
In which case why is it so catistrophically wrong for the cases to be painted ?
When the Australian army flogged off WM20's in the 60's & 70's a lot of them had painted cases.
Some were silver frost while others were green in varying shades.
Similar when they flogged off the B40GA's nearly all of them had painted bars, painted exhausts and painted cases, again done in silver frost but that is not how BSA made them.
There were some B 40's that had green painted cases as well.
Down side is camera's are prohibited on army land so one can not take refference photos of how the bikes looked the day they passed from military ownership to private hands.
I think most of us like to depict our bikes in a wartime configuration and no doubt unit markings were often applied once the bikes were received by their respective units. I don't know how common it was for engines to be painted in REME workshops during the war. Certainly it was common practice post war. So in any case you would be depicting your bike as a REME workshop rebuild with a painted engine.
However though. Matchless did build bikes with various forms of painted engine and gearbox casings.