[Phlogiston]

They are plated. The plating wears/oxidizes off then the base metal oxidizes - turns dark.

[Greenhorn]

Thanks a lot for all the input guys. I didn't realize there had been so many replies.
I ended up cleaning them good with a wire brush and then shot them with a coat a satin. They look ok.

[EdHolland]

Has anyone tried re-plating? I know it is OTT, but there are a brush plating kits available. It would be a lot of work for something that is hidden away.

Greenhorn - that chassis looks great.