Just look it over real good -- nothing wrong with that! Hopefully it's as you say and reasonably solid, especially if it's been in Florida most of its life. It may have been driven up north on a couple winter trips and not washed off, with a damaged or missing wheel well liner (assuming that model had liners) and got a lot of salty slush just in the one side. It happens! Or too many beach runs and that side got more slung up into it. Hopefully that's the case.
I've seen stranger things -- rear pillars rusted through and the rear roof with lots of flaking surface rust UNDERNEATH in a 64 Rambler wagon that came from FL. The fellow used it for a fishing car for years. I believe what happened was he had buckets of fish with a little (salt) water in the back a lot. On hot days water evaporated and got in the head liner, then condensed on the cool metal roof at night. That ran back and down the insides of the rear pillars...