After a lot of years of experience with this issue, this is what I do:
I use an extremely soft cloth and work out as much of the oxidation as I can with Mother's Mag Polish. I also go over the entire blue lightly to remove oxidation you can't see. If you do this lightly, it will not remove bluing, will give you a nice surface to then wax/oil or protect as you want. You can cold-blue it afterwards to minimize the appearance.
A 15 was my first S&W about 25 years ago. I had it tuned and hard-chromed (the finish was very poor already) at APW/Cogan and it was one very impressive gun. That gun is in really excellent shape otherwise and a re-finish or anything excessive would be overkill. Having just the cylinder done runs the risk of mis-matching the blue as salt-bath batches can vary in results. It would very likely be "good enough," but I think again overkill.
Typical cause for a mark like that is long-term storage without any handling/wiping down. Someone put it away with a bit of perspiration or something on it, and then it sat that way for a period. Very common on guns not used or handled much.
Quote:
You guys have me all wound up now. There's a beautiful Smith 67-1 sitting in a pawnshop for $450.
If it is really clean, that's a really very, very good aksing price and you should go haggle about it.