This is the canteen with special cap for US military hydration systems such as the US M17, M40, and M45 gas masks with a Type M connector for canteens and bladders..  One quart potable water capacity.  Olive drab color made of sturdy plastic, does not collapse.  This item may have been previously issued and may show signs of use, but are in excellent condition.  The connector will not work with the Israeli or Soviet masks on this website, nor will it work with any mask with a built-in hydration system.  Will not work with a US M50.  The special cap does not alter the normal functioning as a canteen, you can still use it as you would any canteen.  See accompanying photos for example of it’s application.  Gas mask or hose are not included as displayed in the photos, see other listings for those.