I commented on this briefly, on the "Eye Candy" blog, but what's getting to be the rarest mustang at a show, is one which hasn't been accessorized by it's owner (s). Save for perhaps the Boss cars, a large percentage of Sportsroofs and Mach 1s were built and shipped with white sidewall tires! Go to a show sometime and just try to find a Mach 1 with white sidewall tires, let alone no wing, no side stripes, no "Ram Air" hood, no chin spoiler, no dual exhausts....yet, so many were delivered without.
I used to have a 1930 Model A Ford, and was a member of the local Model A Club. Almost every car in the club was over-accessorized well beyond the way the car came. Sure, you could have gotten some of those optional, or dealer add-ons, dual side-mount spare tires, bud vases, white wall tires, etc, but probably 99% didn't, from old photos of the era. Our Mustangs are the same way.
I've done it too....my Mach 1 has a wing, white lettered tires, ram-air, side stripes...I did all this in '90 when I bought it, figured I was making it special. My car looks better to me, but it's not how it was built, and I'll be honest in telling the next owner ( if I ever sell it ).Today, if I Google " 71 Mach 1 images", of all the photos that come up, only ONE has the standard hubcaps and beauty rings, so today, it seems a car that is authentic to how it was delivered, without owner added options, is RARE.