Regardless of cost, most early plating was nickel (at least for most Gottlieb games I've owned). Mike Chestnut (Horsepower Unlimited) has done a great job for myself and many others. He's pretty busy, so if you use him, make sure you allow turn around time.

JR, If you are talking about metal era side rails, they were made from stainless steel, which is much more resistant to rust than other plating (nickel or chrome) I have seen pitting on stainless, but it takes a lot of prolonged moisture to make that happen. Notice those stainless coin doors have very little problem with rust.