On a slightly different note, reductio ad absurdum(reduced to the point of the absurd), then a car is really defined by its cowl panel and the hidden VIN there? Couldn't that also be changed out for a reproduction one? Or by using an intact cowl found in a scrapyard?
It depends on the definition of "defined".
From a titling/registration view, if the VIN tag has not been tampered with and matches the title, the car is legal. In a case like this, where the body style does not match what is on the title, the two hidden VINs, engine VIN, and drivetrain VIN would be subject to inspection if anyone were to call into question the validity of the car.
If the entire cowl and firewall including VIN, cowl tag, and hidden VINs were cut from a car and grafted into another, there is an excellent chance that no one would be able to tell if the workmanship was good and the car was in paint or primer. It's only in the extremely rare cars that this level of inspection is done on a regular basis. A repro cowl would have no numbers so they could be restamped as anything. I've seen cars with the numbers filled, and I definitely consider that suspicious.
It has not been possible to purchase parts of a car containing the VIN tag legally for many years. Not to say that it cannot be done, as it certainly can if you deal with junkyards that do not comply with scrap laws or with private parties.
One of the easiest ways to spot a retagged car is by the mismatches from the original build. On Mopars, it was easier as the fender tags listed all the options and included the VIN. Our PHS reports take the place of the tags. If your report shows the car had disc brakes and the car has drum brakes, chances are there is something wrong as no one would downgrade to drums. Same with four speed to auto, convertible power top to manual (not likely someone would remove all vestiges of power top), power steering or brakes to manual. It's worth questioning such changes and inspecting the evidence.
