FDE HK 416, MR556 & 417 Flip Up Rear Sight (Diopter 3mm Sight). Attaches to any standard picatinny rail. No hardware needed as this is a self contained quick attach rear sight assembly. This unique HK designed sight features a quick & easy push button attachment for ease of use. For use with a 33mm sight line folding front sight. Can be used on custom AR15 builds or other similar platforms. Rare & hard to find.2mm ? rear sight bore, 33 mm sighting line.