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The bottom of the S-IVB stage as it was displayed in the Rocket Park.

Note the mount points for the main tunnel, which had been removed during the restoration.

Near the upper right corner is the aft end of what would be the auxiliary tunnel. The round hole is presumably an attach point for a cold helium sphere mass simulator. (This particular S-IVB is the dynamic test vehicle, which means that it was built differently from a flight stage, including the use of mass simulators in place of certain flight hardware; refer to the Model Specification, Saturn S-IVB-D Stage for additional information.)