Why does it matter? You run the risk with a State inspection of them issuing a new VIN number to it. Leave it all alone.
I think with a chassis number that low it has to be a '71?
It may have to do with the original Letter of Origin. If it was a US car, Ford issued that. If it was imported then there could be the source of the mistake, because that's what it is, a mistake.
Remember that with some 74 US Panteras, they were re-designated as '73s by Ford because of California Emissions laws. When you buy, you go with the chassis number. There is enough information on that available now to guide you through identification the cars manufacture.