This is a TROY Sight known as the BattleSight, model SSIG-FBS-FMBT-00.
Durability and dead-on accuracy make Troy BattleSights(TM) the hands-down first choice of Special Ops and Tactical Users Worldwide. Troy's M4 Front Folding BattleSight is easy to install and deploy" with no levers or springs to fumble with. No tools required for adjustment.
Features:
A stainless-steel cross-locking system ensures sights remain upright and zeroed under extreme combat conditions
A push of a button folds the sights securely out of the way until you need them again
Each click adjusts .50 MOA