It looks like that because that's as far as my finger fit down the tube with a cloth to clean the dust off the muzzle. Vent hole? I know it has a fuse hole....I would fire a wad of nothing first and start with lite powder loads but I would certainly be a trying kinda guy. If it is a repro they even reproduced the fill holes...
My father got this one while on a trip to Gettysburg back in the 80's. I dont know where he got it. I had another one I shot a few times and a much smaller one I shoot often. To the best of my knowledge this one hasn't been fired since before my father got it. I have not fired it....yet
That will shoot a golf ball nicely. Use the cap from an Edge shaving cream can. Fill it level and dump it in the barrel. Use toilet paper for packing and a cup cake paper cup to seal the ball.
I build cannons as a hobby and 1 3/4 is my preferred bore. I posted pictures here a few years back of a cannon I was building.
Curious to see how this plays out. Would love to have it myself.