To be honest, Fabrizio, I did not find the coach builder.
I found the photo on three different Spanish websites and each site gave a different clue to the identification of the car.
One of our Spanish members who may have access to the SEAT archives may be able to complete the answer, that site was the source of most of my information. I doubt that it was a "factory" prototype only because of the "styling" of the paint scheme etc.
I hope someone else can add to the solution.
Bill