Wheel covers, center caps, trim rings were packaged and "put" into the trunks. Non-power radio antenna masts were sometimes put on the inside of the deck lid with a wide piece of inexpensive tape. Valve stem extensions were in the glove compartment, in a small plastic bag. Dealers put all of those things onto the vehicles during "Pre-Delivery Inspection" or "Make Ready'.
I'll agree that "the black" on the grille was probably to protect the plastic chrome of the grilles. On Lincoln Mark IVs, I was surprised when I was told that the chrome grille shells were plastic. We found some on a L-M new car lot, that apparently hadn't been through make ready that had this hard tar-like finish protector on them. I believe it washed off? Underneath was a chromed plastic grille shell.
Some neat pictures! Thanks for posting.
CBODY67