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The LOX supply 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 there are four externally-visible tube inlets within the LOX supply port.

Note the orifice plates on the four tube inlets, which controlled the amount of LOX admitted to the heat exchanger (and thus determined the amount of GOX pressurant produced). The orifice plates were calibrated for each engine and, on some engines, the orifice plates bore the engine's serial number (although the plates on this particular engine do not).

The text next the flange at the 10:00 and 5:00 positions read "LO₂ IN".