Similar but much smaller signal cannons from the same foundry recently sold at auction. It appears the roman numerals in front of the touch holes are a sizing mark. And not representing the poundage of shot, found on some cannons.
https://auctions.morphyauctions.com/...LOT528250.aspx
"Lot consists of a pair of Swiss iron signal cannon tubes. Both cannons with raised reinforcing rings and a heavily swamped muzzle. Smaller cannon has a single rod marking, probably designating it as "1", while the other has two rods, designating it as "2". The larger measures 13 - 1/2" from cascabel to muzzle, and 3 - 3/4" across the trunnions, with a .75 caliber bore; the smaller measures 11 - 1/4" from cascabel to muzzle, 3 - 1/2" across trunnion, .75 caliber bore. Both cannons have an intertwined "HF" logo between the trunnions, standing for Hallefors Mill, a known foundry which produced other cannons. CONDITION: Both cannons have a pleasing chocolate patina with evidence of casting throughout. Hallefors crest softened but visible. Bores are clear of obstructions and air flows freely to muzzle. A well presenting pair of Swiss signal cannon tubes. Final prices include buyers premium: $676.50"