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The nine-pin plug was an attempt to address a safety issue related to traveling with the refrigerator operating on propane and was used from 1985-88. My understanding is that its purpose was to insure that the gas flame in the refrigerator was extinguished when the tow vheicle's inginition was turned off (such as during a refueling stop) - - and a circuit was built in that did not permit the flame to be relit for 25-minutes after it was extinguished.
The wiring diagram for this plug can be found at:
Airstream Nine-Pin Bargman Plug Wiring Diagram
While I am sure that many would just switch to the current 7-pole plug, if the rig were mine, I would keep the safety device in-tact - - but I am also one of the few who still has working seatbelt interlocks on my '75 Cadillac Eldorado!
Good luck with your trailer!