Part of the standard issue US Molle II system: 1 quart canteen pouch. Also doubles as a general purpose pouch. 

These are in ACU/UCP (that grey digital camo that blends in with a gravel pit and grandma's couch) 

This will fit a 1 quart canteen (and cup if required) but can be used for a myriad of other things, and will fit many NVG protective inserts. 

NSN: 8465-01-525-0585

They feature a flap with plastic buckle and two molle straps on the back. Additionally, they also have two small pockets on the side. They can be used to carry water purification or other sundries. 

These will be from a variety of makers, but all are genuine US Government issue and in excellent shape.