Here is my swing at the start of PT.
I think Deuce is close - In my opinion what we refer to as Pro-Touring today would involve taking a popular and/or available car (as opposed to starting with a upper level car – i.e. Corvette, Ferrari, Porsche) and applying State of the Art aftermarket parts to it to increase the performance envelope. The modifications are done to personalize the car for the owner.
If you agree with this definition, the first PT car could be the Blue Maxi Camaro built by Car and Driver magazine in 1969. They took a (even then) popular car, a 1969 Camaro SS/RS. They added a high horse power small block motor to keep the nose from getting too heavy (the then exotic Corvette LT-1). The latest tires - Goodyear Polyglas G60’s and big stiff American 200S wheels, big brakes (JL8 4-wheel discs) and created a car that was “A 4 wheeled Saturn Rocket for your driving pleasure”.
Their goal was to create a Grand Tourer, inspired by Trans Am racers. Not far off from what we are doing today.
Tom