The problem with using part of the body for the back block, aside from the milling machine cutting that part of the body to chips when the rear of the receiver is cut away, is that there would be nowhere to screw the pump arm rod into that back block.
You are right about not needing that slot, though. You'd be better off without it, in fact.
You are right about not needing that slot, though. You'd be better off without it, in fact.