Just FYI, one of my 70 Mach1's was sold new with a dealer added shaker and 15" Boss 302 wheels. It was a Basic off- standard equipment car that sat on the lot for almost a year (weird color combo, medium gold/ginger (mustard outside, poop inside)). The dealer added the wheels and the shaker to finally sell the car. I know this cause I bought the car off the original owner and have the original bill of sale. The wheels also have a shipping label to the original dealer dated several months before the first sale that's still present when you unmount the tires. The wheels are actually dated about a month before the car was built, so they were sitting around for about a year also. My 68 GT fastback was a really original/unmolested car when it was wrecked and stuck in a barn in 1973. It still had an original tire as the spare mounted correctly in the trunk, but it was mounted on a chrome rim and according to the Marti report, my car came with painted ss wheels.
I bought a new GT convertible and swapped the wheels with cobra wheels I got from the dealer as soon as I got the car home. I guess my point is that a ton of cars were being changed shortly after being sold, or sometimes even before they were sold new. On the flip side I'm sure a lot of mistakes happened at the factory but it's going to be a really tough sell to convince someone it was built that way by ford. A good friend of mine was a parts manager at a local Lincoln Mercury dealer in 68/72 and he told me they were always getting cougars with small screw-ups like a mustang horn pad or door panel that they had to correct. His best screw-up was a 70 Cougar 351 car that wouldn't shift right from day one. after pulling the valve body apart twice, someone noticed it had a N case rear. The car was accidently built with a 4.30 Tlock rear instead of the 3.00 geared rear it should of had. The good part was that my friend ended up with the parts after 30 days or so. I still have the axles, and a friend still has the center section.
With your wheel dates, there's a good chance that the dealer either needed your wheels for something else or the nice wheels were stolen early on, and your car ended up with plain wheels, especially if all the dates match. I'd put the styled steel wheels back on , their a lot nicer anyway (But might take some hunting to find....)
I'll stop rambling now....
John