This is a SB Tactical Stabilizing Brace known as the SBM4, model SBM4-02-SB.
The SBM4 Pistol Stabilizing Brace is the evolution of the original SB15. Thinner" lighter and with enhanced cheek weld ergonomics" the SBM4 provides increased stability" improved accuracy and usability. US veteran designed and proudly made in the USA.
Features:
Fits AR Pistol Buffer Tube
Includes brace and adjustable nylon strap
Diameter compatibility 1.1-1.25