I spent a while wrestling with the beast today to make sure it had preponderance, and it does, it is around five lbs. breech-heavy when balanced on trunnions. There are no marks on the bottom of the cannon. The thing I call the cascabel could I guess serve as a breeching loop, although on the right side it does have some rough casting spillover or something that keeps that side of the hole from being round, as BoomJ noticed. I can stick my thumb thru the hole but it is tight. I thought it was where a bolt went thru to connect something but since the edges of that hole are radiused, maybe it is for a rope, dunno. I also noticed again how the trunnions are on the low-line, unusual for what is obviously a 19th C. cannon (built-in percussion lock tells me that, post-1838 or so.)
I am still mystified. It might just be a kind of unique form of swivel gun, with the big blade sight and percussion lock being "selling features" to beat the competition, but it is so different from all the rest...