Interesting thread. In addition to the 12 ga guns coming normally with 11 ga tubes, the 10 ga came normally with 9 ga tubes. This according to The Parker Story. I have five pre 1880 10 ga guns and a similar number of 12 ga guns of the same period. Every gun I own measured exactly to the 11 ga and 9 ga specs, although I have had a few of them cleaned up. I have no doubt that the gun was originally a 12 ga with 11 ga tubes and was rebored by Parker to as close to 10 ga as they thought safe. They obviously did not get to the full bore dimension but did rechamber.
An interesting side discussion is whether they targeted 9 ga or 10 ga. They ended the practice within a couple of years of the gun being made. The date of the conversion isn't listed unless I missed it. I would presume they would do the conversion work under their current system of using correct bore sizes. If it had been attempted when the 9 gauge tubes were the norm, they wouldn't have been able to come close to the standard.
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