Have a 65 Belvedere that is undercoated inside the doors and there are 2 metal plugs on the door backs where it was sprayed in. Anyone know if this was this a factory deal or an aftermarket dealer applied or a Ziebart, Rusty Jones kind of thing. The holes under the plugs are kind of crude which made me wonder. Silver plugs I'm talking about in the picture are up near the top even with the outside door lock and other is lower, even with the rubber bumper.

my 65 doors were not undercoated by the factory

my 65 coronet has the 2 metal plugs in the doors like yours. my car doesn't have any undercoating, in the doors or floorpans, the plugs are factory.

My 64/5's have those plugs. Think they are to access nuts for the window regulator.

Thanks for the info guys, much appreciated.

It was a common practice for dealers to sell added undercoating and rust prohibitive coatings to customers (Ziebart being one of the best known brands). The process often included drilling added access holes and adding plastic or rubber plugs to fill them after the material was sprayed in.