Thanks DSMITH great idea.
This is each side of the brass.
You found it. Now to deal with it. I would start with some 400 wet and dry with a few drops of oil placed in a slotted stick and rotated by a hand drill. It might just need some minor sanding.
If this is something that makes you uncomfortable doing yourself go see a gunsmith and show him your brass. I have seen my gunsmith do this on several occasions.