Here in Ocala the National Parts Depot (NPD) has a 1967 Coupe that is claimed to be an "all original" car. When I asked them why the Coupe has Fastback ribbed panels on it their excuse is the ribbed panels were a dealer installed item on the car. The car is set up as an "Exterior Decor" or "Exterior Trim Group" vehicle with the styled steel wheels, the wheel opening mouldings and the pop-open gas cap too. I then asked them, don't you think the dealer that supposedly installed them would have known the difference" they had no answer for me. I then asked if I could look at the Buck Tag (because the hood stays closed on the car) and they said no. I figure it isn't original.
Don't get me wrong, I paid good money for a pair of original Coupe panels for a 1967 Mustang that I installed just about every factory option in back in the 80s. I liked every option I put on the car but I always told people the car was a "Make-Me" GT Coupe (not GT-A or GT-Automatic either). I had a factory cruise control unit for the car but I didn't install it because the four speed cars didn't come with cruise. I couldn't find the remote trunk release for it either. It had everything else from disc brakes and a 4-V on the 289 to a working tilt/tilt-away column, working convenience control panel, 8000/140 mph yellow-line factory tach cluster and Kelsey-Hayes original disc brakes. I wish today that I had never sold the car.
Oh, yes, the ribbed rear panels were NOT a 1968 factory option from what I had heard many many years ago. All 1968s that have them have dealer installed ribbed panels. If anyone knows something about this that is different than what I have been told I would like to know though. I am always learning.
(edit)The ribbed panels contour or follow the lines of the trunk lid. The Fastback panels have the wrap-around feature at the trunk key lock because the trunk lid doesn't drop down like the Coupe and Convertible trunk lids do. If you are installing the taillight panels on a car like the one pictured above (California Special) it does look like the Fastback panels would be the correct choice.