This is the special cap for a standard plastic US military canteen that allows you to utilize the hydration systems of US M17, M40, M45 gas masks with a Type M connector for canteens and bladders..  This cap is intended to fit only US military issue canteens, it will not fit any canteen.  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, see other posting below.  Never issued in original packaging.  Does not alter your canteen, you can still use it as you did before.  See accompanying photos for example of it’s application.  Gas mask, canteen or hose are not included as displayed in the photos, see other listings for those.