Congratulations, you have surprised me. I have seen every single one of those devices added to an M-4 or clone somewhere in the world (IRQ, AFG, BHG, etc) except for one.
I have never seen the extension down from the butt plate. I can guess what it is for, but I have never seen one used.
It could have been a beer can/soda can opener but with those self opening cans I think it’s just a orphan attachment now.