This is a Phase 5 Weapon Systems Hand Stop known as the Mini, model MHS-MLOK-RED.
The Phase 5 Mini Hand Stop MLOK (MHS-MLOK) is an ultra-lightweight rail attachment that will assist the operator with indexing their support hand while deploying the weapon; as well as keeping the support hand in the same position while firing. The MHS-MLOK can be mounted on the bottom or either side of any M-LOK compatible rail system in a forward or reverse direction" allowing either a push or pull style grip.
Features:
Red finish
Compatible with M-LOK rail systems
6061-T6 billet aluminum