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The oxygen return port on the heat exchanger.

Internally, the heat exchanger had four oxygen coils through which the LOX passed, evaporating and expanding into gaseous oxygen (GOX) in the still-warm turbine exhaust gases, so (although difficult to see in this photo) there are four externally-visible tube outlets within the oxygen return port.

The out-of-focus text next to the flange at the 9:00 and 7:00 positions reads "GO₂ OUT".