This is a Magpul Industries Stock known as the CTR, model MAG310-OD.
The Magpul CTR (Compact/Type Restricted) - Mil-Spec Model is a drop-in replacement buttstock for AR15/M16 carbines using mil-spec sized receiver extension tubes. Designed for light" fast action the streamlined A-frame profile avoids snagging and shields the release latch to prevent accidental activation. The CTR also features a supplemental friction lock system that minimizes excessive stock movement for enhanced weapon stability* and an ambidextrous QD sling mount that will accept any push-button sling swivel. The Mil-Spec Model includes a standard 0.30" thick rubber butt-pad which provides positive shoulder purchase to prevent slippage even with body armor or modular gear.
Features:
Mounts on milspec-sized receiver extension tube
Shielded release latch
Removable butt-pad
Sling mounts