Midwest Industries ERS Flip-up Rear Sight, Flat Dark Earth The Midwest Industries Emergency Rear Sight (ERS) is a folding, A2-style, dual aperture rear sight that mounts to any flattop AR-15 upper or M1913/Picatinny-style rail. The Midwest Industries back up rear sight uses a detent button to lock the sight in the up or down position. The Midwest Industries flip up rear sight windage knob is identical to that used on the AR15/M16 A2 rear sight. Constructed of hard anodized 6061 aluminum. Specifications: -Flat Dark Earth color -Material:6061 Aluminum -A2 style dual aperture -Standard AR15.M16 style windage knob