First of all, I appreciate everybody’s input. I want to learn as much about this gun as possible.
As I said, I know very little about firearms, and I assumed that one had to be rifled in order to be called a rifle. So am I understanding correctly that a rifle can be either smooth bore or rifled, and the same can be said for a musket? And if so, what is the difference between a rifle and a musket.
Also, does anybody know what the “No. 23" means?
A bit more background: I bought the gun from the family estate auction over 35 years ago. As far as we know, It had been sitting untouched behind a door in the attic of the old family farmhouse since before the 1920s. The farm house itself had been virtually untouched for that long, as well. Our bachelor great uncle lived there, and truly it was like stepping back in time to go into the house when we visited him. For this reason, I’m inclined to believe that our ancestor's gun is an “antique gun in wrecked condition”, rather than one that was cobbled together at a later date. Or maybe that’s just wishful thinking on my part.
BTW, the overall length of the gun is about 60 inches, and the barrel itself is 44 inches long.
Below are some additional photos. As you can see, it is in pretty sad shape, but it must have been a real beauty when it was made.