Have older toilets in two bathrooms, probably early 1960's models of American Standard. A dark,grayish stain is developing towards the front of the trap holes. It's not on the interior surface, but looks to be coming through from the back or bottom side of porcelain. Any ideas about what this means? Is the material failing? Is this just a cosmetic issue-just looking bad? Or a sign of needed attention? Only one "water incident" a couple of months ago, when the older water line gave way and starting leaking in the wall of one bath. That was repaired the same day. Stains really started before this anyway. DH hasn't wanted to replace or update these before this, that's ok. Thanks in advance for any answers.