Part of the standard issue US Molle II system: 40mm High Explosive Pouch (Single). Aka the single 40mm grenade pouch.
These are in OCP/Multicam.
NSN: 8465-01-580-2756
These have a single pocket with a top flap closed by a metal snap. The pocket is tapered, with an opening on the bottom, and is meant for 40mm HE grenades.
These will be from a variety of makers, but all are genuine US Government issue and used condition.