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The LOX supply port on the heat exchanger. The text at the 6:00 position reads "LO2 In".

Note the orifice plates on the four connectors, 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 bore the engine's serial number. The lower-left plate reads "F 2003" and the other three read "2003".

These four connectors correspond to four tubes on the heat exchanger interior.