After the disaster at Dunkirk, the War Department was desperate for equipment, including motorcycles. They would take anything the motorcycling industry would put out, mostly civilian machines painted in military colors. Eventually the government wanted to standardize things and in the end of the summer of 1940, all players in the industry were developing “experimental lightweight prototypes” for approval. Eventually Triumph and BSA were asked to make 50 motorcycles each, presumably for further testing. But during the Coventry Blitz of November 14th 1940, all the Triumphs were destroyed, together with the drawings and the tooling.
From there on, the details are sketchy, but it appears a "shotgun marriage" was arranged by the government between the motorcycle builders, which ended in the above displayed Royal Enfield with a BSA WB30 engine. The full story is in the WD motorcycle forum.