This is a Magpul Industries Accessory known as the Cheek Riser Hunter/SGA, model MAG461-GRY.
The Hunter/SGA Cheek Riser Kit is designed for use with the Magpul Hunter X-22" Hunter X-22 Takedown" SGA Shotgun Stocks" Hunter 700" Hunter 700L" and Hunter American stocks. It allows the user to configure their firearm with the ideal cheek piece height to accommodate a wide range of sight/optic configurations and shooter preferences. The Riser Kits are available in two configurations" Low and High. This High kit includes one (1) 0.50" cheek riser and one (1) 0.75" cheek riser. Made in U.S.A.
Features:
Includes one (1) 0.50 cheek riser and one (1) 0.75 cheek riser
Allows ideal cheek piece height
Accommodates a wide range of optic configurations