I bet you mine came with the carpet delete is why the trim is missing. There is no sign of carpet ever being back there. I believe there was once that black cardboard/felt type stuff.
That's the first I heard of a carpet delete option. I'm sure it is true though.
In your case I can't see why the fold down seat back would be carpeted, and not the rest of the cargo area. If the plywood looks original with no signs of screw holes in it, you may have the proper answer (delete option).
Too bad the build sheets are usually missing or destroyed before we get the cars. It would make it easier to know what was correct for our cars. Previous owners almost always make changes to the cars.
The seller was trying to sell mine as a Formula S. It even has a Formula S badge on the trunk trim panel that is not correct for the car. I checked the VIN before the purchase.