It appears to me that the variation in your barrels is just the result of using a ribband that had a differing composition of iron and steel. Whether this was intentional or accidental is a matter for conjecture.
I'm interested in what you said about rough barrels having a coating of sulfuric acid applied before shipping. I'd assume this would have had to have been neutralized after bringing out the pattern, or all you'd see is a coating of heavy rust.
It has always been my understanding that what Miller says about the proof process is essentially correct. I believe the tube are rough bored for the provisional proofing, but they are not chambered. But I have never been able to find out exactly how the proof houses breeched or contained the proof charge in unfinished and unchambered barrels.
