This is a Limbsaver known as the Recoil Pad, model 10019.
The Limb Saver AR-15/M4 Snap-On Recoil Pad securely snaps onto most six-position adjustable stocks to reduce felt recoil by up to 60 percent while reducing muzzle jump. The recoil pad helps increase the accuracy of follow-up shots and you'll enjoy increased control for maximum stability. It's made from LimbSaver's proprietary NAVCOM technology (Noise and Vibration Control Material)" which effectively absorbs a wide range of frequencies to dissipate energy and vibration. The no-slip surface stays in place" even in the rain. Quick and easy to install" this Snap-On Recoil Pad is durably built to be used outdoors in all harsh environments.
Features:
Snaps onto most six-position adjustable stocks
Reduced felt recoil by up to 60%
Reduces muzzle jump allowing for faster target re-acquisition