Now that is a nice rifle, Ed & in lovely condition.
Love those fences.
Do you have it in hand yet?

Seems to me I recall reading here about a forum member having the heavier SxS 12bore meant for the heavier 7 dram load, but that the case was also 3" in length. When I was shooting round balls in a SxS 12 I had, I was using a 2 3/4" plastic hull and getting the full 7 dram load in it along with bore sealing "cupped" wads. LOTSA recoil - too much recoil for the 7 1/2 pound gun I had.

This looks, by the fences, to be a round ball gun. Might not be, but that is the impression I get.

W.W.Greener gives 1 round ball load for 12 bore, using a 547gr. ball. The calibre of that gun is listed as .725" with 110gr.(4 drams) "powder" for a 7 1/2 pound gun & producing 1,384fps.

A different load of 7 drams, ie: 191gr. "powder" and 599gr. projectile (I assume short conical) with gun weight of 13 pounds & producing 1,584fps.

The cross section of an 8 bore round ball load, shows powder, then a thin card wad, then thicker fiber wad and another thin cupped upper surface to hold the ball, then the case crimped into the ball.

OH- do I see NP on the bottom of the barrels?
Appears so. If so, then Lyman Shotshell manuals give 12 bore data for round balls as well as conicals up to 525 or so grain weight.