After the Mk 3 came the ETA Elite with a larger rear main ball race which meant the front housing casting was modified to have a bulge in this area. Rear venturi size went to 5/16in but was often cut away and threaded to accept a Cox type carb. as is the case here.

The original Phillips Head 6BA screws used for retaining the front housing and rear backplate have been unobtainable for many years now. The trouble with the replacement types is that the head diameter is just a little too big and they don't look right and interfere with the sides of the front housing