Part of the standard issue USMC MOLLE FSBE system: Bandoleer, ammunition pouch. More commonly known as the mag bandoleer, or that shoulder pouch that holds all the mags.
These pouches are all Coyote Brown, not Khaki.
Each cell will hold two M4/M16 magazines.
NSN: 8465-01-516-7961
These feature a shoulder strap, though they can also snap into the top pocket of the standard issue Assault pack, which is a neat feature. They have three internal cell pockets, with a metal snap closure on a flap.
These will be from a variety of makers, but all are genuine US Government issue and in excellent shape.