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Overall view of the Service Propulsion System (SPS) oxidizer service connector (used to fill and drain the Service Module's SPS engine oxidizer tanks). This portion of the Service Module extends into the Spacecraft-Lunar Module Adapter (SLA), and is serviced via an opening through the SLA. On the CSM/SLA on Johnson Space Center's Saturn V, there is a plate covering this connector.

Unlike the launch vehicle, whose propellants were loaded via the Launch Umbilical Tower (LUT), the Service Module's propellants were loaded from the Mobile Service Structure.

I've prepared a page diagramming the Service Module's aft face, featuring the aft heatshield and some of the Service Propulsion System (SPS) plumbing, including the propellant fill/drain connectors.