Toby,
I regards to the caustic blueing issue. When I see barrel flats black over, or a "too" shiny finish over pitting, or a purple tint, I start looking harder. At this point I typically find completely blacked bores, lumps blacked on the narrow side etc. All of this points to caustic treatment of the barrels. Many are not yet shot mind you. If I get no air escaping when the barrels are dunked in a tank of hot water, I simply take extra care to clean things up, then finish properly. If the solder is shot, it's either relay ribs or scrap them, depending on value.
My point is that many of these signs are relatively easy to spot, and that I've seen a good bit more of this than you seem to expect as the norm.
Jim
