GG&G A2 Flip Up Rear Iron Sight.  Detachable sights are a great option with or without optics.

This GG&G sight comes either as a standard flip-up with a locking detent or as a spring-actuated flip-up with locking detent. The detent release is on the left hand side so the off hand has quick and easy access to let the sight down.

Black finish available.