This is a tank used in the Gemini capsule to supply drinking water to astronauts. Made of fiberglass and metal, it has a rubber bladder inside. The tank used oxygen as a pressurant. It is marked "Non-Operating Use Only" and thus was employed solely for testing purposes. Airesearch made this artifact and NASA transferred it to NASM in 1968.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.