Thanks, folks. I put a saturated boresnake in and pulled about 2 inches every five minutes, while working on another project. When done, I pulled it through five more times. The spot I saw before is gone, and I'm a happy camper I guess it was just crud.

BTW, the bolt appears to be filled with grease, and there was grease filling the the ball at the end of the bolt handle, and around the barrel throat in the receiver. The first pass with the boresnake picked-up some from inside the barrel, too (might have been the spot I saw. Is that something people do when they store for a while, like a D-I-Y cosmolene?)