This is a Knights Armament Company Rail known as the Free Float RAS, model 20208.
The KAC Carbine Free Float RAS (Rail Adapter System) features full length mil-spec M1913 Picatinny rails at the 12-" 3-" 6-" and 9 o'clock positions" making it compatible with any number of rail-mounted tactical accessories. The rail free floats the barrel" allowing for consistent barrel harmonics upon firing" and thus increased accuracy potential. Includes three KAC 11 Rib Rail Panels.
Features:
M4 RAS (Rail Adapter System) Forend Assembly
Free float design
Mil-Spec M1913 Picatinny Rail
Carbine length compatible
Three 11 rib rail panels
Fits AR-15's with Carbine Length Gas System