This is the tank inside the Gemini VI-A spacecraft that supplied water to astronauts Walter M. Schirra and Thomas P. Stafford. Launched on December 15, 1965 on a two-day mission, Gemini VI achieved the first space rendezvous ever with Gemini VII. The tank has a rubber bladder and used oxygen as a pressurant. It was removed from the spacecraft after the mission.
Airesearch made the tank, 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.