Can't find the reference, but I seem to remember that they were a mid-year addition to '67. It's a change in the structure of the top bracing over the quarter window--impossible to add later.
The early shoulder harnesses were kluges--there were two buckles. You would have to modify the inner quarter structure to get a later-design self-retracting three-point belt setup on there--and the belt would always be hanging there.
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