Whitey,
I have no better idea than you, on the rear latch. However, after studying the pic with the back of the magazine showing, it appears to me that there is a rectangular mortise across it. As well, what appears to be the magazine lever, might conceivably be for another application here: that of latching the mortise on the back of the magazine assembly, to some sort of lug on the front of trigger guard.
I note that the 'slant' of the magazine & the front b'll lug are oriented for the unit to slide to the rear as it is inserted into the stock. That would tally with some sort of hoizontal acting lug/mortise interface at the rear.
All just deduction, of course. Mebbe some other contributer can pick up where Frank left off, with detail pics.
