First I would suggest the title of the thread be changed to "1967 and 1968 ......."
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A few pictures I hope may clarify the findings and question
These are the two areas on fastbacks we seem to be focusing on
Typical 67 fastback interior rear side upper panel . Showing the area asked about in the original post (green arrow) and the front seat seat belt anchor for Mustangs (red arrow)

Close up of both areas we seem to be focusing on. The forward seat belt mount typically would not receive a threaded anchor plate in the hole indicated by the red arrow except when (rarely ordered) the front seat shoulder belt was ordered. Same hole and anchor plate installed for use for the roll bar upper attachment point. In neither of the examples is a plate installed in the pictures.

Typical 68 fastback interior rear side upper panel . Showing the area asked about in the original post (green arrow) and the front seat seat belt anchor for Mustangs (red arrow)

Close up of both areas we seem to be focusing on. Front shoulder seat belt anchor (for Mustangs) was also used for the upper roll bar attachment point as in 67
