Norbert..
There was never a full size luggage rack on a fastback,78-82, Corvette.
I believe if someone ordered a T-top rack system as an option with a new car there were holes in the rear bodywork to plug in brackets. These holes had rubber inserts and you installed brackets with pins that fit the holes.
The T-tops fit onto the brackets and got strapped-down.
I wish I could find pictures to prove this!,
Graham