John, are you referring to civlian M20s or the British military variants ? With Nortons, it's easy as the WD prefix indicated military but BSA confused matters by chossing the "W" prefix for all the 1940 model range, including the planned and produced civilian models. As Rob mentions, the WD version was something of a hybrid.
The WD KM20s are often referred to as "deluxe" but it's not entirely clear what "deluxe" referred to. I think it was Michiel who was of the opinion that the deluxe models had higher-grade forks. The catalogues make it clear that they certainly had QD rear wheels whereas the standard model did not. It seems that official WD contract KM20s had the valanced rear guard and QD rear wheel, but no valanced front guard and no tank panel. Perhaps, in the light of the MEE trials, the QD rear wheel was an essential feature and the machines were referred to by BSA as deluxe.
For 1940, for the civilian market, only the QD rear wheel, fully valanced panel tank model was available so essentially only a deluxe. There were some emergency purchase panel tank WM20s delivered to the War Office during late 1939 but never with valanced front guards.
It seems likely that any late 1939 civilian sales would have featured the valanced front guard, as did the Dutch contract machines.