Documented original numbers matching: "This is the holy grail. Now you have a car that was born with these parts, restored exactly the way it came down the assembly line, and you have documentation to prove it. Probably the loosest form of documentation would be notarized previous owner affidavits - letters from previous owners that say this car, when they bought it, was a four-speed car. The next step up is dealer paperwork, like copies of the dealer invoices when the car was originally sold. The kicker is, all of this stuff can be forged; I have seen some brutal forgeries out there. Then you get into factory documentation - warranty cards and the original body buck tag. Of course, there are people who can make these things seem absolutely genuine, so there's this red-flag, buyer beware thing saying that just because somebody has this stuff doesn't mean it's original or authentic. The next thing is the build sheet. A genuine build sheet has all of the numbers and codes so it's the ultimate documentation."
Third party authentication: "This is the fourth leg of the stool. I call it the sprinkling of holy water. You now have people like Kevin Marti who can do the numbers analysis and tell you when a Mustang was built. You have the Shelby American Automobile Club to document Shelbys. I have a lot of respect for these third party verifiers. When you think about it, they have placed themselves in harms way to try to assist the hobby for a greater good."
Is Your Mustang "Numbers Matching?"Stamped Numbers