Thanks for responding, guys. Fernando, you sound like you're closer to expert than you let on!

Your comments are right on and that I didn't have the funds to get this cannon before the auction ended (got a nice boarding axe instead, though!)
Kronckew, thanks for posting all of these pics of some pretty cool cannons. That trunnion-less one in particular is interesting to see, as most either had them or were the older signal-types. I had never seen one without them until you posted this pic. I had thought deck gun, but of course they were all swivels, so your argument stands. Yes, in the early/mid-19th century, they started using a percussion firing system on deck cannons, ship signallers, and even on some swivel guns. I guess it was a lot easier than trying to use the old lit fuse.