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The helium supply port on the heat exchanger.

Internally, the heat exchanger had two helium coils through which the helium gas passed, expanding in the still-warm turbine exhaust gases, so there are two externally-visible tube inlets within the helium supply port.

Note the orifice plates on the tube inlets, which controlled the amount of helium admitted to the heat exchanger (and thus determined the amount of pressurant produced).

The text next to the flange at the 10:00 position reads "He IN".