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Overall view of the aft end of the SA-T stage.

Picture 2 of 2.

I believe the silver panel with the round connectors on the tail unit just aft of the white LOX tank is one of the short cable mast umbilicals. This is the one located at Position II, and a second short cable mast umbilical would be located on the opposite side of the stage, at Position IIII (sometimes appearing as Position IV). The Position IIII short umbilical mast can be seen connected to its umbilical plate, just left of the "SA-4" markings on the tail unit, in this unidentified picture over at the Project Apollo Archive.

The short umbilical masts provided electrical and pneumatic service required for ground checkout and operation of the stage before liftoff.

As the short cable mast's name might imply, there was also a long cable mast. The long cable mast, the red vertical umbilical running parallel to the white LOX tank near the center of the stage in another unidentified picture at the Project Apollo Archive was used to service the instrument compartments located above the propellant tanks. However, the long cable mast was replaced by a swing arm umbilical carrier starting with SA-3, the third Saturn I launch. Although it was really only necessary to differentiate between the "short" and "long" cable masts for the first two launches, the short cable mast retained this name throughout the Saturn I and IB programs.

Read more about the short umbilical mast starting at page 95 in the PDF and read more about the long umbilical mast starting at page 102 in the PDF in the Umbilical Systems, V-2 to Saturn V (direct link to 11.5M PDF).