This is a Knights Armament Company , model 25460.
The KAC Terminator Hand stop provides a tactile reference point for the support hand that helps the shooter to acquire a consistent grip on the rifle. It features an integral QD Sling Swivel Socket on its face for increased versatility. Attaches to any M1913 Picatinny Rail surface via a screw and clamp arrangement. Push button sling swivel sold separately. Aluminum construction.
Features:
Aluminum construction
Attaches to 1913 Picatinny
Integral QD sling swivel socket