They were natural-finish black rubber. Most guys use some sort of cleaner or "bumpre-black" stuff. This may buid up over time, I guess. If its been sprayed with paint, you should be able to safely strip it with paint-thinner solvent; just don't soak the rubber or you'll pull the softening agents out, leaving it brittle. They're already 30+ yrs old and getting a little hard.
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