English: IU at IBM plant in Hunstville. Note the +Y marking, which implies that this IU was made subsequent to the change in vehicle coordinates effective for IU-204 and IU-502 and subsequent missions. In this system, X is vertical at the time of launch, and the IU bears Y and Z markings. In the older system, the IU had X and Z markings. ^[1]

Original description: This view depicts engineers conducting a system test on the Saturn V instrument unit (IU) at International Business Machines (IBM) in Huntsville, Alabama. IBM is a prime contractor for development and fabrication of the IU. The IU is vital to the proper flight of the vehicle. It contains navigation, guidance, control, and sequencing equipment for the launch vehicle. Three-feet tall, twenty-one feet in diameter, and weighing about 4,000 pounds, the IU is mounted atop the S-IVB (third) stage, between the S-IVB stage and the Apollo spacecraft.
ID: MSFC-6861934

Credit: NASA Marshall Space Flight Center (NASA-MSFC)