Thanks for posting the picture of this interesting weapon. I haven't seen one just like it, but pending your finding some additional information, I'd guess it was a mid-19th C. insurance gun, possibly made by Cyrus Alger of Boston MA. Alger made many civilian insurance guns for sale to merchant ship owners, who were required to maintain a certain level of armament aboard their merchantmen to comply with Lloyd's requirements for underwriting.
If you look at old admiralty court proceedings, the first item they addressed is establishing what the armament of the vessel had been at the time of loss, in terms of carriage guns and swivels.
Alger borrowed features from US Navy guns it had made, and put these details on many different patterns of small cast-iron cannons for private parties. Yours is different from any other Alger insurance gun I've seen, but that's not surprising to me considering the many different patterns of them that exist, scattered in private collections, on monuments, etc.
You should look closely at the ends of the trunnions because Alger usually marked their weapons in that location with "C. Alger Boston" or "C.A, & Co. Boston." They would also usually mark the weight of the gun as a whole number of pounds (not in CWT) near the breech somewhere.
Never having seen this model before, that's my best guess for now.
To Join the Company of Military Historians click here