The turning of the barrel appears to be original. I don't see any machine marks of any kind. The threads are indeed a v-groove and rather deep. the extractor cut-out is a v-groove as you describe, and it appeared to be where you describe in relation to the top flat.
I was thinking that with all the 'Cowboy action' people out there, somebody might have an interest in this cannon barrel. I'll bet it packs a whallop! I looked in the chamber, and I don't even see any entrance ramp or rifling wear. if this was reblued, somebody did it a longgggg time ago and did one helluva job. I've heard of the Ballards, they were legendary at the Creedmore range shooting competitions, with Harry Pope barrels, if I had one of those I'd be keeping it! lol
I'd be interested in selling it, I was going to put it on e-bay today and a few other of the online sites. I just picked it up today and have been researching it. I wasn't the only guy at the estate sale ;((
It wasn't cheap, but I'd be happy to net $50 bucks on it and give it to somebody that knows what it is.