Trijicon MRO (Miniature Rifle Optic) Mounts Fits the Trijicon MRO and MRO Patrol Mil-Spec drop test rated 7075-T6 aluminum design Installs with 4 screws and hex key (included) Lower 1/3 co-witness means that the MRO is mounted higher than the iron sights resulting in a clearer field of view; The iron sights remain visible in the lower 1/3 of your optical window for use as a backup Distance From Top of Mounting Rail to Optical Axis: 1.663 in.