Commonly known in the West as the LRBHO device:
Looks very simple in concept! Latches against the hammer pin and not the trigger pin. Engages the bolt stop which I assume is spring loaded to stay in the down position.
There is an obvious gap toward the top of the magazines where the rounds begin to slide forward, and there's just enough room for that front "finger" that is bent down into the magazine to be engaged by the follower. That seems to be how all of this works out. On an aftermarket mag, that corner will need to be cut out some for the finger to fit. I'm not sure how well this would work WITHOUT the magwell. But it does raise some possibilities.