I think it is a 66 I wouldn't try to clean it any more but you could wax it it could help it from falling apart and bring out some detail Dig until your arm falls off
I put the coin in a spoon and put the spoon over my gas burner for a few seconds to get the coin hot then I put a little white candle on it to melt over the whole coin. While the coin is still hot I take it out and rub it with a rag so that all that's left is a thin film of wax.