This is an image taken from "Classic Motorcycle" showing a Norton WD16H outfit in RN service. Based on the unusual "pannier" tool boxes, it must be from contract C.6653 or C.7353 - mid to late 1940 - The machine does indeed carry a Middlesex registration.
Civilian number plates seem to have disappeared early on from machines carrying War Department census numbers. Did the Air Ministry and the Admiralty carry on using them ?