I have started cleaning a Junghans box Clock from about the 1920's there are what appear to be stains or marks on the dial. I have seen this kind of staining before, on other clocks. I am aware that aggressive cleaners can remove the numbers, and metal finish. Were these dials anodized or plated? Would a slightly acidic solution work? Vinegar or lemon juice mixed with distilled water? I wonder if this is not perhaps nicotine or oil from peoples hand that has caused the problem. Attempt a light clean or leave as is?