I belive Arne is on the right track. Spectrographic grade Acetone is what is often used to clean lens elements during overhaul (I used to do this for a living). Just be careful as he suggested. Acetone will remove paint and worse yet, dissolve the cement that holds elements together in groups. Just a little at a time applied gently with a Q-tip should do the trick. The stuff you get at the paint store will be sastifactory for what you are doing. Also, extinguish flames, cigars, etc. and don't breath this stuff a whole lot.
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