Went over to an old Mustang guys house today.  He's been into them since they first came out, and has owned hundreds of them. He had a 66 red vert that was covered up in his garage and had boxes of parts stacked on top of it so I couldn't get a good picture.  He said it was made by Ford and at one time was going to be an option for convertibles, and that only 10 or 12 were made as prototypes.  The pics don't do it any justice, it looked like a very nice piece.   It's a "Roadster" option like the ones you see on convertible TBirds.  It cover up the entire rear seat area.   Very nice.