First - forgot to welcome you to the site
As I understand it (correct me if I'm wrong) you are not referring to your or someone else's specific car but instead are asking if some of the cars received the incorrect spare tire decal rather than the specific decals Ford wanted the workers to apply at the factory. Yes this sometimes happened, workers mess up. This could this could and likely did on one car or a group of cars. This could be an individual worker that just grabbed the wrong one or a worker that shift just installed the same decal on all the Mustang before some inspector caught it and had him change his ways. Basically to more pay attention and put the correct one on the cars as outlined. It just happened.
To support the idea that a specific car was built that way you would need to collect pictures of the original trunk lid and decals on the original paint along with the stamping date and other supporting documentation. Then f more than one car built at the same time and plant also had the same detail finding and documenting those might help your case if you choose to go that route.
Of course that would not be an easy task to try and document if there was a group of cars especially given the condition of most of the cars this many years later and the number that have also been restored erasing many of the original details.
In these cases (as we typically suggest for "mistake cars") its always best to place or reproduce what was expected, what was typical or as Ford designed and wanted unless you have good documentation. Without it you won't or typically won't "get by" at a show plus you will also mislead many that view the car. They will often assume that the detail is correct for all similar models and in turn you will be responsible for their seeing and following your lead.
If you have a car with an oddity it can be a challenging choice but one you must make and understand the ramifications of your choices. And as we've seen you'll become the guy with "that car"
