most of the vehicles sold east of the rockies were from the janesville wisconsin plant. I think they started rusting right there at the plant
at my dads salvage yard I remember 567 chevys that were "local" to the area, one would be a oakland/l.a. car, and one would be a janesville car, the california built car would be rust free, and the janesville car would be rusting away.
it has pretty much been my experience that if there is a "J" in the vin, there is rust.