As good as the weather-sealing might have been back then, compared to modern vehicle designs, there's no comparison. Dust (from roads and atmospheric dust) will filter into areas inside the doors and such and settle out (for a long time). In there, ambient humidity can be absorbed, along with outright water getting past the outer glass fuzzies/weatherstrips In , will make the accumulation damp and foster rust from the inside out. In doors, this usually happens at the bottom of the inner structure on the door skin. It might take 30+ years for that to happpen, but it eventually will if the accumulation isn't removed and the affected metal treated to stop what's there and prevent it going any further.
Looks like that rubber "flap" at the forward bottom of the quarter window is a bit loose to seal very well.
Just some observations,
CBODY67