Did you notice the left side of the magazine has a shallow curved lip which extends along much of it's edge, that must also barely clear the bolt? Notice the shine along the magazine edge from the bolt rubbing it during cycling. The right side of the magazine is pretty typical looking. I would assume this is a feeding improvement for this particular cartridge preventing the rear of the cartridge from popping out too fast. I once did something similar with a 444 converted No.4 Mk.1.