It's an RC25C anti aircraft sight. Was made originally for the 50 cal HMG. It's poo though. The sighting system is too complicated. By the time you work out which part of the sight needs to be aimed at the target to score a hit the target would have fucked off hours ago!
It's dog poo. At least the SUSA is x4 magnification where this has none. On this sight once you have acquired the target and worked out the direction of travel, speed and distance you then have to work out which of the many lines and dots on the sight is relevant to hit the target. It doesn't lend itself to quick target acquisition. Much easier to use the lead or ambush method using the SUSA.