This is a Meprolight Sight known as the Fiber Tritium Bullseye, model 0631013108ML63101G.
MEPROLIGHT combat proven self-illuminated night sights enable you to hit stationary or moving targets under low-light conditions with dramatically increased hit probability. Designed as replacement parts for the standard weapon sights" MEPROLIGHT'S Self-Illuminated night sights can be mounted directly with minor modifications. Superior production methods and rigid quality control have made MEPROLIGHT the first choice among law enforcement agencies" defense decision makers and major weapon manufacturers.
Features:
Always Ready When You Need Them
Daytime Confidence Under Low Light Conditions
The Brightest Night Sights Available Today
Totally Integrated Day/Night Sighting
Maintenance Free