There was a carpet delete option, there's a '67 here in town with that. It's got this black cardboard/tar paper stuff back there. He's also got fold down seat delete....yup, the rear seat does not fold down. It's fixed.
I never heard of that and I dont see anything listed in any of my reference material. In 69 The fold down seat became an option and the black material sounds like what they used under the carpet.