This is the Agena-B upper stage used during the 1960s as an orbital injection vehicle for Midas and other satellites. It was also an intermediate stage booster for Ranger and early Mariner space probes. Made by Lockheed, it was fitted on the Thor or Atlas-D launch vehicles that became known as the Thor-Agena and Atlas-Agena.
Most notably, Agena-B also served as the Corona photo reconnaissance satellite which then flew under the cover name Discoverer. Agena-B used a restartable and gimbaled liquid-fuel rocket engine made by the Bell Aerospace Company. This Agena-B was transferred from the U.S. Air Force to the Smithsonian Institution in 1965.