As a unibody the interior panels would have receive a coat of red oxide color primer sealer. Over that you might find some overspray from the grey surfacer/primer but that would be limited normally to the lower panels near opening. Overspray is effected by gravity
The the door jamb, trunk and exterior color was applied which would produce a little or allot over spray - much more on panels and surfaces near openings were the painter would have sprayed the edge (part of the paint going on the edge the rest into the interior) and those panels directly behind. Plus the paint "fog" would have drifted downward onto the lower panels
Since the inside surface of the roof is inverted and faces downward much less overspray got to its surface. So (as a estimate) it might show 90% red oxide when viewed
Here is a partial shot of the finish under the headliner and padding - in this case a coupe but both body styles would have been treated the same way.
Do have one example of a June 68 Dearborn fastback where the painter took a little extra time and sprayed the bottom of the roof panel with the grey primer sealer over the red oxide but IMHO not typical so not including that one off example
Hope this helps
