This is a Sig Sauer SBX Gen 2 Pistol Stabilizing Brace.

The new generation of a pistol brace that kicked off a revolution, Sig Sauer SBX Pistol Stabilizing Brace is a revolution in small arms for American civilians.

Featuring durable, NFA-compliant construction, the SBX is designed to help stabilize one-handed shooting while using an AR-style pistol or something like the Sig MPX. Manufactured to close tolerances and built to fit any pistol/attachment with a buffer tube, the Sig SBX creates exciting new possibilities for avid shooters.