There is information on the 'Historical Facts' page on this site.
Here's an excerpt taken from "British Forces Motorcycles" Well worth buying a copy.. Ron
During the latter half of 1943 the girder-fork steering-damper was removed from all new machines, the crankcase sump-shield was redesigned to become a pressed steel item incorporating additional protection at the sides for both the engine and frame-rails, and the fuel tank was altered by having the rear offside corner removed to accommodate the substantial hose section leading from the tank-top mounted Universal War Department pattern Vokes air filter to the carburetor air intake. This last modification was carried out on a large proportion of machines destined for service in hot, dusty climates. By early 1945 the Vokes filter was fitted as standard to the fuel tank of all new models irrespective of where they were destined to serve, the filter being secured to the tank by mounting-strips attached to the previously redundant knee-grip locating
