Peter many years ago you and I discussed a red "GT5". I believe it was in Spain. It may have belonged to a forum member. Or it may have been for sale on eBay. I really don't remember any longer. I've searched my records for a picture of it, but came up empty handed.
I'd be surprised if you didn't retain some information about it.
The unique thing about the car, it had factory original GT5 body work, but the factory ID plate on the foot box described it as a GT4. The red "GT5" on the cover of Jan Norbye's book is described on page 72 of that book as a GT4. That Pantera in Spain could have very well been the Pantera on the cover of Mr. Norbye's book.
From 1978 to 1980 wide body Panteras came on the scene and evolved from a "wheel, tire, and fender flare" option for the GTS (1978), to a unique model so equipped (1979), and finally to the unique GT5 livery displayed in Turin in 1980.
1978-GTS/4 → 1979-GT4 → 1980-GT5
The red Pantera, by its existence, proves that the evolution of the livery outpaced the evolution of the name. I don't know if in your mind the first GT5 is the first Pantera called a GT5 by the factory, or if its the first Pantera built with GT5 livery. Either way, I doubt there were many Panteras like the red one. That Pantera's chassis number would have pointed you within a few chassis numbers to the "first" GT5, however you define it.
If there is any car more unique than a GT5 its a GT4, which is also an Embo coach (9000 series) model.