This is a Magpul Industries Stock known as the PRS GEN3, model MAG672-BLK.
The PRS GEN3 is a field precision stock for AR15/M16 and AR10/SR25 platforms" featuring tool-less length of pull and cheek piece height adjustment. With solid adjustments for length of pull and cheek piece height via aluminum detent knobs" the PRS GEN3 (Precision Rifle/Sniper) stock provides a stable interface and is intended for semi-automatic sniper or varmint type rifles. Offering a nearly universal fit" it is optimized for rifle-length receiver extensions but will also mount to many mil-spec carbine and A5-length tubes*. Includes a cant/height-adjustable rubber butt-pad and rotation-limiting QD sling swivel cups as well as M-LOK(R) slots on the bottom for rear monopod mounting. Made in U.S.A.
Features:
Machined aluminum LOP and Cheek Piece adjustment knobs
Steel adjustment shafts finished with Melonite
Bottom M-LOK slots for sling or monopod