This is a Midwest Industries Sight known as the Flip Rear Sight, model MCTAR-SPLP.
The Same Plane Low Profile Rear Sight is constructed of high quality 6061 aluminum" hardcoat anodized for a lifetime of service. The rear sight folds from the rear and locks in the raised position using a ball bearing pivot detent. A micro style windage knob features the same .49 MOA as on M16 rifles (.65 MOA on M4 rifles).
Features:
Sits only .4375 when folded;Folds to rear;Extremely low profile dual aperture design;Locking detent with large surface area for maximum strength;Micro style windage knob featuring the same MOA adjustments as the standard M-16: .65 MOA on M-4 type rifles / .49 MOA on M Type rifles
