That is the military tnspectors acceptance stamp. Absolutely WD. Each inspector had his own Identfying (3 digit) number. In your case it's miss stamped and only showing th #2....Which was quite common.
Here is the inspector in the Welbike factory. Later in the film that this still was taken from, you see him stamping the frames.
Possibly your engine was a spare engine, hence the plate. I expect Jan will know. I'll be interested to see what the plate says. Ron