The few pre-War Flea's (the civvy "Royal Baby") and then the wartime produced WD/RE's had big-end rollers 1/4" diameter by 21/64ths long..........the post-war civvy versions had the roller length increased to 1/2".................
So, the pre-war and wartime crankshaft assembly were slimmer than the post-war type, and as such, the individual components are NOT interchangeable although the complete crank assemblies ARE...........so you can put post-war crank assemblies in earlier cases and vice-versa.........:-)
As for the crankcases themselves, the wartime were cast with sparcer internal "webs", particularly on the drive side, therefore were marginally lighter in weight........post-war, the crank-cases were "beefed up" with reinforced castings...............cases however, ARE interchangeable..............
Incredibly, I have the RE factory blueprints dated 1943 that show the modified (post war) crankcase and big-end assembly but not adopted, it seems, until 1945 when post-war production commenced..........