[The359]

I agree that it does appear to be a non-functional prototype, most likely just to use as a concept car on the auto show circuit to help continue interest in the 86. There's so little room between the tunk lid and the rear seats that the rear seats would have to go if they were going to fit a soft top.

Many car companys build one-off convertible prototypes that never make it to production. I remember when I had an Oldsmobile Alero, GM had a convertible version built that never made it beyond the concept stages.