The early cars had a metal tag, just like other Fords of the later 60's, attached to the drivers door by the latch mechanism. Later in the 1st year it was the foil sticker attached to the body below the drivers side striker plate.
If your car has the original door and it is missing, you will see the 2 rivet holes where it used to be. If the door was swapped out the tag was probably tossed with the old door. Marti can make new ones if you want one.