This is a Trijicon Red Dot known as the RMR, model RM05.
The Dual-Illuminated RMR uses fiber optics and tritium to illuminate the reticle. Fiber optics collect ambient light" automatically adjusting the reticle brightness. In situations with no (or low) light" the tritium illuminates the reticle. This version is popular with shooters who prefer a battery-free solution
Features:
Dependable electronic design
LED RMR
Button lockout
Versatile sight
Parallax free
Unique patented housing shape
Battery conservation mode
