Mil-Spec M4A1 vertical grip
This is made by P&S Products, who produces accessories for military weapons. This item is identical to the Knights Armament KAC vertical grip, but is marked with the P&S CAGE code. P&S Products, along with Colt and Knights and other manufacturers all build to DOD spec for M4A1 weapons systems. P&S Products also makes the M4 quad RAS and the Colt M4 buttstock.
Note: Ignore logo in the pic.