Did the metal tags appear in 68 or were different methods used in different assembly plants.
No earlier and believe it was a standard plate/blank supplied by Ford to all plants. Example below would have come from the bottom half of the door tag after it was stamped, used for the temporary owners card or simply (in early years) just glued to the inside cover of the owners manual. As we know the upper half (silk screened portion) was attached to the drivers door.
For an example like this normally a strip of owner info was placed where only adhesive remains in the picture made with Dyno Tape a popular label making tool of the period. Discussed in another thread
I would like to see owner cards from different years and plants.
The application/use of the metal tags started in 65-66 will have to see how early I can find. As mentioned and I think shown in at least one other thread those were attached to or placed in an envelope in the front cover of the owners manual. The permanent owners card came later and its believed they (the permanent or in later years the temp one also) to the clip in the glove box
Guess your looking for the temporary owners card (with the metal portion) not the others which were all plastic