This is one of a dozen small mirrors placed in the aluminization tank when the flight mirror for the Hubble Space Telescope was aluminized. While the mirror remained in storage, periodically one of these witness samples was removed for testing the integrity of the reflective surface.
Sky & Telescope Magazine donated this object to the Smithsonian in 2013.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.