i would try some soapy water and a soft bristle tooth brush, i would not press very hard just in case the letters start to come off. i would think that would be the least aggressive way to clean them.
I had a set of 1937 ford gauges and after disassembly soaked them in vinegar and than used a q-tip very litely to wipe the face clean it worked great for me. Now these are anodized and painted and yours maybe different so I would only try it on a spot that will not show first.
Mike
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