Last year I went to the Mustang Owners museum in North Carolina. All of the mustangs there are either the museum owners personal stock, or lent by mustang owners around the US. Stock changes every 6 months or so and they even have one of the oldest (by VIN that weren’t showcase preproduction) mustangs as a “barn find”. 
The people that run it are obviously knowledgeable and were a blast to talk to. When I went they mentioned that they were trying to acquire the full scale clay S550 model. It’s possible that it would have been the one that was used in the documentary “A Faster Horse”.
The wikipedia article!
And I was able to use the listed sources at the bottom for more info.