The washers under the rivet nuts were the tooth lock variety, so I would guess that they were zinc coated, but I do not know that for sure.
If I remember right, the earlier 1970 cars had a deck lid that only had the provisions for the luggage rack.
Then, once the rear spoiler became an option, another decklid was added to the mix.
Once the T/As came into prduction with the ducktail spoiler, they had a deck lid with provisions for all three options.
Every 1971 deck lid I have seen is the same so far, has provisions for all three.
It changed again for 72, not sure after that...............
Hopefully someone here has photos of all the 1970 types?