Sorry if this is garbled but I'm still learning computers. DD has been a great help.
The piece you have in mind, seems to me to be in the class of a TARASNICE or palisade gun even though this illustrates one on a trestle in the 15th c.. A "tarasnice on wheels" is in the Tocnik castle inventory. Some are cast in bronze and 133 cm long (over 4') and 45mm (1.5"). My guess would be that you are in the range of 1.5 to 1.75 caliber and 4&1/2 to 5 feet long barrel with 28" wheels.
Narrow single trails were hardly uncommon in the 15th c.. The thicker are for houFnice.
The hole on the trail may either be for an elevation post like the Mary Rose or to receive a limber pin (doubtful but possible?).
Sometimes we have to think out of the box. I was associated with a group that would not deviate from what they observed in a 16th c. painting for clothing; as if everyone always wore the the bedspreads and drapes and never went casual. Let's face it: people were painted in their finast and only the finast could afford to be painted in their finast. So! Now we come to cannons. (Boomer says, "It's about time").
Barrels wether bronze or iron were valuable and recycled into the fashion of the time. Your cannon as someone noted, may not be on an original carriage, to which I will add, may not have originally had trunions; may once have had a different stock. Because we have never seen exact examples, doesn't mean variations did not exist. You can't tell me with any authority, that the Duke of Paduca never had a barrel painted blue.
I am an advocate of recycling and independant thought.
Richard "The Beatnik"