Hi Mike - beautiful bore rifle. Thank you for sharing so much great info. I thought you might find these dimensions interesting for comparison...I have a very nice William Ford chamberless 10 from the 1920s. Full coverage engraving. Lovely gun. So it has 8 bore barrels and 10 gauge chambers, proofed for 3.25 thin brass cases and 2.25oz of shot. Barrels weigh half what yours do (4.95 lbs) and barrels are very thick through to the muzzles.

Here are some additional measurements:

at the breech, total width across both barrels is 2.592"
height of right barrel at breech from flats to 12 o'clock: 1.365"
height of left barrel at breech at same point: 1.374"
barrel thickness at breech, measured just inside the cut for rim: 0.243"
diameter of rim relief cut: 0.936"
diameter of chamber just inside rim: 0.862"
width across both barrels at muzzle: 1.874"
height (diameter) of barrels at muzzle: 0.961"
thickness of barrels at muzzle: 0.085"

The barrels are 30" long. Total weight is 10.2lbs.

More info at https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=276052&page=1