This is a F.A.B. Sight, model FX-RBS.
The RBS is an advanced" dual aperture" durable and low-profile auxiliary rear sight. It can be used with any front sight and was designed for more accurate aiming and faster target acquisition. Windage is easily and quickly adjusted without tools. RBS is readily mounted on any 1913 Picatinny rail" making it ideal for use on M4 / AR-15 / M-16 platforms and on the FAB Defense(R) K.P.O.S.
Features:
Can be used as a primary or back-up sight
Ergonomic angel-wing shaped front shroud
Low folded profile
Duel apertures are 3mm longer
No tools needed
