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They were just painted. To me it looks like a semi-gloss black. I don't know if the original paint is still available, I'm sure the NCRS restorers know. Last time I looked, I saw that GM has a black frame/chassis paint for sale. I've bought several spray cans of semi-gloss frame paint from Eastwood. It looks good to me. They advertize their paint as having a high solids content. (I think that means it's a thick paint.) Have a look at eastwood.com.
I have found some asphalt like substance sprayed on parts of the rear suspension of my 68. For instance , there's some on my trailing arms. I don't recall seeing any asphalt like stuff on the front suspension. I have a NOS early C3 frame that I bought from GM. It only has paint on it.