Part of the standard issue US Molle Close Combat Set: The 50 Round Mag Pouch. These are meant for use with the SAW and therefore unofficially called "saw pouches"
These are in OCP/Multicam.
NSN: 8465-01-F05-4527
These have a single pocket with a top flap closed by a buckle. The pouch is square to fit a 50 rounds of ammo for the saw.
These will be from a variety of makers, but all are genuine US Government issue and like new condition.