I agree that it is very unlikely that the chains were painted. I doubt that they were zinc plated. Many parts on these trucks were cadmium plated. Maybe the chains, maybe not.
The problem with cadmium plating is that it is a sacrificial coating. As long as it is still there, it takes the brunt of the corrosion, protecting the underlying steel. The problem is that being sacrificial, it doesn't last very long, and rust wins anyway in short order.
I am guessing that all GM was concerned about was that the metal was shiny until the owner got it home.
I am going to do to the chains what I did to the hood hinges, hood springs, hood latch and nuts and bolts.
I have accidentally found out that if you prime with Rustoleum self-etching primer and follow up with Rustoleum silver metallic before the primer completely cures, it runs and streaks and looks awful exactly like what cadmium plating looked like on these trucks originally.
I have a real good idea what the cadmium plating looked like originally when I removed a hood hinge from the cowl.
GM used a sealing putty between the hood hinge and the cowl to keep rain out of the cab via the hinge bolts holes. The result was that the putty preserved the cadmium finish.
