Thanks for the responses, guys.
I LOVE patina (part of why I bought the rifle - the somewhat worn barrel finish), but this sticks out as foreign. I looked at it under a magnifying glass, and it doesn't look like anything is added ON to the receiver (tape, JB Weld, etc). It appears something was taken away from the receiver. If it was just solder or tape, I'd steel wool it off, rub the area with a black powder residued patch and call it a day. There's a similar area at the base of the magazine follower, which is why I'm suspicious about the frame. It looks like the magazine follower was allowed to slam back against the frame, chipping away the finish there as well. If I could just polish it out and/or rub blackpowder residue there to mellow it out to yellow, I would, but it looks like the metal has a completely different finish. Additionally, the fact that I paid for it what I did and the condition was not fully disclosed bothers me on principle.
Got a note in to the seller, and see if he has any insight. I've added a picture of the receiver where the follower hits below.
Thanks for the responses and ideas!
Hasta luego,
Wild Ben