I would not clean that one. Folks like original surfaces. Leave it up to the next owner if they want to clean or not. Cleaning should be done when a problem will get worse, such as the green on the 1968. Maybe try acetone (pure, not nail polish remover) first. If that doesn't work, ask a dealer to do a quick dip for you.
The Mexico km486 20 Pesos (1980-1984) Mayan Culture If you are selling or not I wouldn't clean them, but it is up to you. Follow link below on Omnicoin I have a good collection of coins of Mexico Cheryl
Bob, the meaning of the word LEY in Spanish means LAW. Silver would be plata and gold would be oro in Spanish. Lou