For Chevrolet vehicles:
The engine line inspector wrote the engine, transmission, and carburetor codes on the Broadcast Copy and put the sheet in a box for pickup by a Scheduling clerk (needed to create the P-O-P at the end of the Final Line).
From: http://www.camaros.org/assemblyprocess.shtml#chev

The 65-67 warranty books were imprinted at the plants. There was a perforated page opposite the plate that the dealer was supposed to remove and stick in the car's file. Some dealer's left it in.
At the start of the 67 model year, there was a shortage of the warranty books so most protectoplates were shipped on a temporary card. Wouldn't surprise me if some plates were shipped loose in the glovebox as Charlie mentioned. See the below link for a sample of that card. https://impalas.org/wp-content/uploa...-pops-temp.jpg

Some 68 plants imprinted the plate on the opposite page. Norwood did as well as some others. If the book has no vin punched in it, then the plate was put in that book by the dealer or owner.

A few 69 plants imprinted the plate in the battery box. I know St Louis did it for the fullsize cars and I believe Baltimore Chevelles too. Not sure if that was done for the whole model year.

And the warranty plates with mostly just owner info were mailed from Detroit. I don't believe dealer stamped the plates themselves except for a few who actually pulled the plate off and stamped the owner info into it! I know of one or two that did it that way. That would be some extra work!