I've got a D plate from 86, manufactured in Germany. One of the first ones. The lack of arch liners does account for the shitty condition of non-restored wings around the top corner where it meets the windscreen panel & around the side repeaters.
They did change the metal as well as include arch liners in the later cars so that's why they are holding out better than the early ones.

The chassis legs & floor pan on mine are both ok (bar 2 bits on the floor where it was jacked up in the wrong place , but are repaired very well). Sills are one to watch on all the cars, not just the oldest RSs.