I also have an early Parker hammer gun that was represented as an 11 gauge, but letters and seems to be a 12. There is an article in the most recent Double Gun Journal that claims a lot of Parkers in 1880's or so were made with 12 gauge chambers and 11 gauge bores. I think I have several. They are supposed to pattern better with the open bores, so enjoy your find!

I just checked out a set of 12g. top action hammer gun barrels that measured .760". And both tubes were full choke.

Brain, what were the wall thickness measurements?

Look up Richard Hoover's articles both in DGJ as well as PP on his research on 11 gauge Parkers. They're very interesting. He and Austin Hogan communicated in depth on the 11 ga. subject.

I think the pattern improvement mentioned above, is the key to the different measurements. I don't know much about the subject, but to have measurements for .729 to .760, all 12 ga., it makes some sense they were trying to get the perfect combination.
Has anyone ever found steel barrels with these measurements or were they confined to twist/damascus barrels?