This is a Yankee Hill Machine Co Sight known as the Flip Rear Sight, model YHM-9680.
AR-15 Flip Rear Sight- Features of this sight include: a rugged but lightweight design with a steel base" a quick release thumb screw" and aluminum stem. Locks in the deployed position with ease and takes up very little space on the upper receiver.
Features:
1/2 M.O.A. and comes with a standard rear peep aperture
Mounts to any picatinny rail
Designed to co-witness with the standard M16-M4 front sight
