Your barrel length equates to exactly 75 centimetres, so perhaps they were made in Belgium originally before being proofed.

I have a Jeffery hammer gun with 30" barrels and proofed as a 13/1 bore. In the catalogue it is listed as the Farmer's Gun, so with thicker barrels to avoid dents. How much do your barrels weigh? The Jeffery weighs 1540 grams (53 7/8 ozs.) whilst my similar 30" Webley 12 bore only weighs 1440 grams, or 50 3/8 of your ounces.

HB