Well, dealership maybe a stretch, but I would encourage you to research it more closely. First you will need to determine the exact dealer and then see if you can find some old newspaper advertising
Since the Cobra name was used on the Shelby its unlikely a Ford dealer would have used it on a Mustang.
I think the flag looks similar to the 1972 Olympic Seont, so there may be a connection to the other modifications
I would put my money on a creative owner