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At the aft end of the engine is the engine base closeout, which has some sort of white, quilted insulation on it.

Turbine exhaust is discharged through the base closeout, so it would seem that these opening are for that purpose. The larger opening appears to be approximately the size of a J-2 LOX turbopump (modified versions of which were used for the XRS-2200), although the bolt pattern appears to be different than an unmodified J-2 LOX TP.