On display at the Smithsonian National Air & Space Museum’s Udvar-Hazy Center in Washington D.C., it’s an Agena-B:
I told you most of the Agena story ages ago, but let’s talk a bit more about the Agena-B here.
Agena-B was fundamentally a stretched version of the -A, with an improved engine (with increased thrust and the capability for restarts in space) and twice as much propellant capability. This made it an ideal launch vehicle upper-stage for taking spacecraft to high orbits (like the Midas early-warning satellites), and to the moon (Ranger).
Notably, Agena-B also served as the base of the Corona photo reconnaissance satellite which at the time flew under the cover name Discoverer. This Agena-B was transferred from the U.S. Air Force to the Smithsonian Institution in 1965.