Hello, you probably found the old article on B&P, now on someone's blog, that gives pretty much the story on that firm. Your gun looks to me like a mid-to-late 19th C. piece, smooth lines and all. A muzzle ring, as you put it, is merely part of the style de jour on any cannon, at least in my opinion. It is possible that some countries at some brief periods of time made it some kind of code for something but I don't think there was any such thing going on with these commercially-produced and openly-sold commodities, the founders only went with what was in fashion, what the customer ordered, or whatever for best business practices.
I had a chance to purchase all the old B&P drawings, all FULL SIZED on linen, from the late Val J. Forgette Jr. but did not come up with the $4k required in time and they eventually went back to a museum in the UK, I don't know which, perhaps search online. Some had notations as to what the gun was intended for, but I never saw the drawings personally.
Your swivel gun was for insurance purposes, not a salute gun, as merchant ships did not normally fire salutes.
