I remember an article or letter written in about 25 years ago (either Mustang Monthly or Mustang Times), where a guy had bought a completely original 1965 Mustang coupe with the High Performance 289 from the first owner. The car had never been apart and was in good condition. Had the Ownercard and owners manual, car appeared to be all correct. But while the door data plate and all of the documentation had a K for the engine code, the aprons had a T (the rest of the info; year, plant, body and consecutive number were the same). Sounded legit, and I have seen quite a number of stamping issues in the years since. The deal with the 67 Shelbys (A in lieu of K and K in lieu of Q) are the only 'group' mis stampings Ive ever heard of. But the Shelbys were probably the only cars built in large groups based on engine type.
Dave