It may be a one-of-a-kind. With the band around it (if it's a separate piece) it looks like a home-build. Still could be quite old, but anything that was going into production would likely (not always of course) be cast. With a cast barrel a large cannon shop would have the larger lathe to turn the trunion, a smaller shop might resort to heating the band and shrink fitting it to the bore. If it was a signal cannon, it would be for a rather large ship, most of the yacht cannon I've seen had more like a 1" bore or were 10ga.