some panels have generic numbers stamped in them, possibly a stock locator number.
Your VIN in the dash should match the hidden VIN if 68 and up. I would look under the package shelf also for a vin that late (73)
Prior to 68 it was an order number that should match the fender tag.
it might read like 72C02123456 72= Hamtramck C = December 02 = the 2 day of and the last are the VON (Vehicle Order Number).
My 68 valiant has the order number on the core and trunk rail. ill need to look at the build date on the build sheet to see how early of a car it is. Both my 69's have the vin on the trunk and core.