Part of the standard issue US Molle II system: Bandoleer, ammunition pouch. More commonly known as the mag bandoleer, or that shoulder pouch that holds all the mags.
These are in ACU/UCP (that grey digital camo that blends in with a gravel pit and grandma's couch)
Each cell will hold two M4/M16 magazines.
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. There is no molle on these, so they work well as standalone items.
These will be from a variety of makers, but all are genuine US Government issue and in excellent shape.