This is the Instrument Unit, Serial No. S-IU 500D-2 as used on the Dynamic Test version of the Saturn V. The IU performed the crucial function of guidance, navigation, and control for the Saturn 1B and the Saturn V: the primary launch vehicles for the Apollo missions to the Moon and for the Skylab space station. A ring-shaped structure, the IU was an integral part of the Saturn rockets, and were located between the upper stage and the payload. (Apollo Command and Lunar Modules had separate guidance systems that took over for later phases of their missions.)
IUs were built by the IBM Corporation, at a plant in Huntsville, Alabama next to NASA's Marshall Space Flight Center. After the last flight of the Saturn (for Skylab), ownership of several IUs was transferred to the Smithsonian.
Transferred from NASA