This is a Magpul Industries Stock known as the SGA, model MAG460-ORG.
The SGA Stock is an ambidextrous" user-configurable buttstock designed to add much-needed adjustability to the tried and true Remington 870 12ga shotgun. Featuring a spacer system for length of pull adjustment" improved grip ergonomics" recoil-reducing butt-pad" optional cheek risers for use with optics/raised sights" and other accessories" the SGA 870 brings Magpul design and engineering to another classic American firearm. Available in Black" Flat Dark Earth" and Orange to adapt to the mission at hand. Made in U.S.A.
Features:
Spacer system for length of pull adjustment
Fits Remington 870 12ga shotguns
Improved grip ergonomics
Ambidextrous
Recoil-reducing butt-pad