Original US Army 2 quart canteens! Holds about two liters, collapses when empty, and all in all a handy thing. Designed for the Vietnam war and has seen use ever since.
Note! The pouch is a Korean (South?) made copy of the original US article. That said, it seems sturdy enough, not bad at all. The bottle is genuine American-made stuff.
Vietnam War finally made US Army realize that a soldier might need more than one small bottle of water, and 2-quart canteens were introduced as a countermeasure. We've got no idea why not call these "half-galloners", but that's the US of A for you. While all previous canteens were rigid, this one is collapsible, rubber-like plastic, so it takes less space when empty. These were meant to be carried in rucksacks but often saw belt use too. Before the introduction of MOLLE and its general-purpose pouches, 2-quart canteen pouches were often used as "stuff it all" bag for various big items without specific pouches of their own.
Both the parts are in very good condition, but you should definitely wash the bottle before use.