this is the iron sight, called a leaf sight, for the Colt or LMT M203 grenade launcher or flare launcher 37mm or 40mm barrels.
mounts to a standard rail on M4 or similar rifle to aim the M203 projectile. Top Rail Mounted Leaf Sight for the M203, 50-250 meters. This is new old stock, meaning the product is new, but has been in storage for a number of years. The manufacture date is most likely 5, 10 or 20 years ago.
NSN: 1010-01-522-0055
Manufacturer's Part Number: KM-22109
Note: Charlie's Custom Clones also carries the LMT Leaf Sight, which is very similar.
Knights Armament (KAC)