Congratulations to:  Jim Bitzegaio
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He identified the mystery part as a COOPER VANE.  After DB Cooper hijacked NWA in Nov. 1971, the airlines installed this gadget which, after takeoff, swiveled in the airflow blocking the tail stairs, and preventing them from being lowered in flight. I believe we have the only one known to have survived. Most were deactivated once security in the US pretty much ended hijackings in this country.