Seems that tanks on machines destined for desert service were "cut" in this fashion to facilitate the routing of a hose from the carb to an air filter box mounted atop the fuel tank.
There are various pictures on this site that show the filter mounting and hose installation...
here is a link to info on the Volkes air filter setup:
The tank-top air cleaner has a connector pipe that runs up where the right lobe would be at the back- so from about 1943 tanks were supplied without a right rear extension.
Earlier, and post-war tanks were supplied as "normal" with both rear extensions.
It should be noted, however, that in the case of the M20 these filters were not fitted during the period of the desert war. The invasion of Italy had taken place by the time the filter was introduced.. The 'cut off' tanks were fitted to the last three contracts of the war only and the filters to the last two contracts only..and many of these bikes went into storage.
There is an existing photo of a Vokes equipped bike in Italy..but so far I think that is the only one showing a Vokes in the European theatre of operations. I have also seen a photo of a Vokes equipped bike in the Far East...Ian