I only have an outdated copy of Peter's registry with me, and I'm travelling, but this car doesn't appear in the version from 17 years ago, which is the only version I have with me.
I've sent Peter an e-mail, and expect him to chime in soon. If it isn't in his registry he'll be happy to chime in with what he knows and be able to include the car as "found", but it doesn't help Jackie with his problem.
I could be mistaken, but didn't Roland end up with all the factory paperwork?
As far as I know, the factory paperwork would be the only way to know 100% if the car was an original GT5, but I'd say that if it looks like a GT5 now, then the probability of it being "authentic" is high. In the early 80's the factory was in full swing producing GT5 cars, and a glance at Peter's registry shows that probably 90% of the cars produced in the early 80's were GT5 cars (as opposed to GTS, the only other option). There were a few oddball GTS cars built, but they were all made to customer order so it would be unlikely for someone to order a narrow body car and subsequently convert it to wide-body.
Anything is possible, but the vast, vast majority of cars from that era were GT5 cars.