It's a 5 round p mag. Will also accept other 7.62 p mags as well as aics mags.
I think it is actually made for the magpul stock, but will fit a bdl with some work. The mag well is about 1/8 inch or 3/16 wider than the factory bottom metal and there are some angles inside that would need opened up. All in all not a difficult install. But I'm glad I tried on a worthless adl stock before something nice. When I buy a better stock I'll most likely reinstall this set up in it as long as I don't have issues until then.