Leon, I've never seen any order or instruction regarding the overpainting of the black panels although many units seem to have done that to conform with later practice.
There are more images of WD16Hs with the BEF showing the black panel and I can't imagine that any markings instructions would have been marque-specific.
Nortons though had been in WD service on a large scale since 1937 whereas the KM20 only appeared in late 1938 so there were simply not as many in service.
BSA seem to have dropped the black panel earlier than Norton. It looks from some of the early photos as if the bikes were delivered with the black panel already applied.
The forum photo shows C398657 from contract C3139 which commenced delivery in April 1939. Below is C3914805 also under new onwership. Slightly later, it is from C3655 which would date it to June 1939 onwards.
At some point during the summer of 1939, the panels disappeared, with Norton also but they mysteriously reappeared after the first 2000 of the September 1939 contract C5109.