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Keeping important bits from gettting caught in the folding top mechanism is the real reason....then they provide a guide for the shoulder belts (optional)
I don’t think 68 or 69 had them even with shoulder belts.
For the convertible I believe you are correct.
Think that they were available for the coupes as "extensions" but they got in the way of the rear compartment doors.
The ones that the OP has in his pic appear to be 70 and up convertible quarter trim panels.
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Slideshow: A Jaguar designer's personal project imagines what a modern front-engined Corvette might look like if Chevrolet revisited the golden age of the Stingray.