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The round, silver fitting is the H₂O₂ (hydrogen peroxide) fill and drain valve, which would connect to the engine's hydrogen peroxide tank.

A hose from the hydrogen peroxide servicing truck would attach here to fill the engine's 76-gallon hydrogen peroxide tank and (for Mercury-Redstone flights) the auxiliary peroxide tank. Another hose would attach to the hydrogen peroxide overflow and vent port, located on the end frame next to the steam vent, and connect to the truck's H₂O₂ overflow tank (basically an enclosed bucket).

Overflow hydrogen peroxide was diluted and discarded, due to the possibility of contamination; 75% hydrogen peroxide required special handling procedures (personnel who handled hydrogen peroxide wore protective clothing consisting of boots, flame-proof coveralls, a face shield, and gloves) and any contamination would make the peroxide dangerous to handle.

Also note the LOX fill/drain valve at upper right.

This view was taken through the steam vent.