Steve,
I made a call today to an interesting man. He is an original owner of a 1970 `cuda convertible with luggage rack. He is a sharp minded individual. He is certain that the car never came with ANY sort of strap. He told me that he inquired with his selling dealer and they recommended that he purchase some through the parts department. He thought that it was crazy that a luggage rack car never had any sort of tied down straps!

At least todays cars come with cup holders...

Steven,
My RT/SE has a rack and I am the second owner.
The original owner saved everything but no straps
Bob