It's a tail rudder.......needed if you want to land this baby in The Peeples Republick of Kommiefornia.
Actually, it's a Kydex grip wrap which makes the rifle "featureless" and thus legal to own in this state. You can either have a pistol grip or a "removable" magazine but not both. Hence the popularity of the so-called bullet button which according to the law makes the magazine "fixed and only removable with the aid of a tool" (the tip of a bullet, that is).
Seems weird? Wait until Hillary is in power and Scalia's successor orders/tells you what's best for you...