I've seen those stains in my two fridges, too. I always thought they were some form of reddish mildew, as I keep one fridge defrosted and the other does it itself. I use Armorall "Ultimate Interior Cleaner" on it, and it disappears nicely.
I'd think rust would be easy to identify. Lick a fingertip and press it against the stain. Then sniff. Wet rust has an ironlike odor that you should pick up on right away.
Also, the paint on the fridge compartment fins of my '75 gave up years ago. As of yesterday, it was still happily making cold.
I really think the best thing we can do for old ammonia-absorption fridges is to keep them running and recirculating all the time.
1975 Argosy 28 "Argosy"
1979 Excella 500 31 "Betsy"
1992 Lincoln Mk 7 LSC
2003 Dodge 2500 Cummins "TowHog"
"Lucy Loosehair" the cat - Airstream mascot
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