Jim
This seems to be an area for research opportunities; and maybe a start would be making a list of guns found with the "XX" and "XXX" barrel stamps? In the past I haven't written down the serial numbers of the guns I seen with these stamps, but a good start might be this gun, SN 30041, which would be 1890 production and features the "XXX" barrel tube stamps, and is also stamped as a Grade or Quality 3. Is this gun recorded as a Quality 3 or "XXX" gun in the ledgers?