By what looks like 8.4 on the barrel, I'll take a wild guess that it's a 12 gauge (18.6 mm). Can you get a photo of the area in front of the barrel flats?
That isn't all that wild a guess!
I'd bet on 12 gauge also. The 18.4 would have been what it measured when it was new, measuring the bore today at a point 9" from the breech will give you a clue as to what kind of mischief has taken place in the century or so since it was proofed.
PS powder was in use prior to 1900, when PT powder came along, but, was used for a few years after 1900, alongside PT.
Best,
Ted
