Part of the standard issue US Molle II system: Flash Bang Grenade Pouch (Single). Aka the single flash bang grenade pouch.
These are in OCP/Multicam.
NSN: 8465-01-580-0211
These have a single pocket with a top flap closed by a plastic buckle. These are meant to hold the ever essential flashbang grenade.
These will be from a variety of makers, but all are genuine US Government issue and used condition.