Herc,
The specification does look very close to that of the War Department machines and a number of civilian bikes were impressed for military service. Perhaps this one missed the Ministry Inspector's stamp ?
The published contract information doesn't include this motor number but there are gaps. Have you found a frame number ?
The saddles have a German look. Has the bike turned up in Continental Europe ? If so, it could well be one of the 20,000 plus motorcycles that the British Forces abandoned in May / June 1940, many of which were used locally afterwards.
Have you tried scraping the paint in odd places to see if you can find any khaki green or, just possibly, pre-war R.A.F. blue ? If it's from a Military contract and it's never been shot-blasted then there must be some original paint somewhere such as inside the headlamp or tool box etc.
Another clue might just be the light switch. If it has a "T" position (tail light only) that could hint at some military history although I'm not certain that early Model Ds would have had the military switchgear.
Rich.