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A fin interface (visible just left of the support post). I believe this is from Fin II, the left-hand fin the booster sits in the Rocket Park.

I speculate that wiring from the electrical connectors on the fins would be routed through this opening.

There are several sets of markings, some upside down, although none of them are fully visible due to the angle from which this photo was taken.

The bottom markings are rightside up and read "Rev. A", "[unintelligible]117", and "Sep 11 1956".

The upper, upside down markings read "Rev [unintelligible]", "MPL5[unintelligible]", "MB56[unintelligible], and 02[unintelligible]".