It could be one of those just cut down so that it won't obstruct a sight rail or something.
I was thinking that but the KAC and SureFire both have legs that extend over the receiver so an optics mount platform can be attached. The one pictured does not have those legs, which would rule out KAC and SureFire.
Also notice the machined cutout around the forward handguard pin. Also notice it almost seems flush with the bottom of the trunion, unlike the B&T and Knights that the rail hangs low. Also notice the step inward at the top of the rail, to not allow visual of the unsightly gap between the cocking tube and the barrel like the knights, and surefire rails leave.