I've been working on a gorgeous Waltham Model 1883 Crescent Street. I cleaned it "by the book" last week and as I was putting it back together yesterday I noticed something odd above the word "Adjusted". At first I thought it was merely something that landed there, but when I tried to removed it with pegwood it wouldn't come off. Then there's the coloration, almost rust-like. So I went back and compared photos from before and after cleaning and to my shock/surprise (horror is the better word) I saw that it wasn't there before my cleaning. I wasn't aware that these plates could rust or oxidize, and certainly not so quickly. I always finish cleaning with two rinses in rubbing alcohol followed by gentle heat drying (blow dryer) and let them air dry (in a very non-humid work environment) for a few minutes before putting them back into the parts tray (about a week ago I cleaned it).
My photos (before and now) aren't the best, but I hope it's enough that some experienced folks might be able to tell me what it is and how best to stop/remove it. I have vinegar and alum, but I'm a little concerned about using anything that might damage the viewable top plate any further than it is now.
Any suggestions or ideas about how to proceed?
Thank you in advance!