The rifling is very shallow, obviously.
I wonder if it was bored out at some time to shoot shot better?
The muzzle is rough, not smooth & shiny like the rest of the bore.
18.76 is .7386", which is loose 12 bore (.730").
If that is the bore measurement, and not the grooves, then the grooves might dictate this as an 11 bore, which is .75",
ie: .19.050mm, groove to groove - AND meant for a brass case with an 11 bore ball. I would measure the bore ahead of the chamber and behind the rifling at the muzzle, if possible. Interior expanding ball and a mic will work at the breech end. slugging up a pure lead ball behind the rifling and pushing it back to the breech for measuring would work for the bore measure.

What a beautiful gun in absolutely lovely shape.

BTW - nominally, a 10 bore is .775", but a full choked 10 bore with 40 points of choke, has a muzzle measurement of .735" or even tighter for some European guns.