English: This photograph was taken during the final assembly operation of the Saturn V launch vehicle for the Apollo 4 (SA 501) mission. The instrument unit (IU) was hoisted to be mated to the S-IC/S-II assembly in the Vehicle Assembly Building high bay at the Kennedy Space Center.
Note that the access door has been removed (about 8 o'clock), to allow technicians to enter the IU after it is stacked. The access door is location 8. Location 7 is to the right, with the nitrogen purge ducts (used during refueling to prevent the buildup of dangerous fumes). Location 6 has the silver cylinder that is the water accumulator. (This water sublimated to space during a mission, providing cooling for the elctronics in the IU.) To the right, at locations 5 and 4 are shelves to hold batteries. This differs from the configuration for IU-514, on display at the NASM. IU-514 has the water accumulator at location 3, to the right of the battery shelves. At the right (4 o'clock), at location 22, is the silver sphere holding the nitrogen supply for the gyro gas bearings. IU-514 also differs in having another battery shelf at location 24, two places to the left of the nitrogen sphere. Note the −X marking on the side facing us (12 o'clock), indicating that the old coordinate system was in use for this mission.