Well, looks like a false alarm. I stopped by a gunsmith on the way to work this morning, and he said he didn't think it was rust. Possibly some debris from the brass or something, not sure... but it didn't look like rust.
He didn't sound totally sure of himself though, so I stopped by a different place for a second opinion (can never be too careful with the pup). This guy said it was definitely not rust, maybe just coloration from the way the metal was heat treated. But he was an old salty dog of a gunsmith, and he was sure. Good enough for me.
