This is what B5 Systems says :
 
 


The same great GripStop for your Picatinny Rails

Invented for Marines and refined by combat use, the GripStop acts as an all-in-one hand stop, vertical grip, and barrier stop. The curved serrated side offers a solid support for pulling your AR to your shoulder, while the flat side acts as a handstop and place to brace the gun against a wall, rock, or window frame.

    Features:
    Mounts to all Milspec 1913 Picatinny Rails
    Made of aircraft-grade aluminum
    Milspec Type III anodized finish