Ya know, from what I understand, the tarnishing/oxidation is just going to happen on the RM bags (atleast from the older seasons from what I hear) More than a few of my bags have this characteristic but ultimately I don't mind. I feel like it adds to the character and gives it a slightly vintage feeling, KWIM? Anyhoo, if it really bothers you, you can very carefully apply some brass cleaner to spiff the shine back up.