This is a Meprolight Sight known as the M-21, model 0626210.
The M21 Self-Illuminated Reflex Sight is used and trusted by militaries and law enforcement agencies worldwide. It is designed and manufactured to the highest military standards to assure years of reliable operation under all field conditions. With no switches" no battery or other external sources of power" the sight is always ready for action.
Features:
30mm Diameter lens
Large field of view
Rapid target acquisition
