If the meter is a VDO unit with a twist off Bezel you can remove the plastic double lens assembly by picking out the O ring holding it in. This only works on the VDO units with the twist off bezel (Round or Square) The two lenses have a small
gasket between them and you can separate them. You can get lenses but they clean up easily with tooth pase or plastic polish. The yellow is sulpher from the rubber
parts that migrate onto the plastic. Some Armorall or other rubber treatment helps keep the rubber gaskets soft and prsvents moisture from intruding. If it is a
single metal bezel it cannot be opened without mashing the bezel. If it is rust it could be a Hobbs meter as they have a lot of ferrous
parts in them.