Part of the standard issue USMC MOLLE FSBE system: The "Pocket SEA Mk 2" pouch is designed to hold a standard NBAVSEA emergency egress air bottle. These pouches can't really fit much else other than some slim water bottles.
These pouches are all Coyote Brown, not Khaki.
NSN: 8465-01-516-8376
These will be made by Allied Industries, all are genuine US Government issue and in excellent shape.