Not much that can be done if you're talking about the little metal plate. The rondelle.
If the green goes as deep as the plate is thick, even a magic wand would do you no good. Poof! Oxidation gone, and the plate would fall off.
If you're talking about STAMP impressions on a stem---like a Dunhill reg # on their early models---a paste of Comet kitchen cleanser powder (lots of bleach in it) pressed into the characters and left for a while, then rinse & repeat a few times, will usually blacken the characters.
You could try that ^^^^ on the rubber around the rondelle, come to think of it. Worth a try. Might lighten the green enough to be noticeable.