Michael and Alexander have posted similare topics on tiller cannons, like the one discussed here
The first thread deals with a earlier but modified example. It was dated by Michael to the 1450-60s. This was before his opus about the touch hole and dating criteria. After this i must say i would date the barrel in question to a 20 years later from the 1470-80s based upon the barrel mouth and the three staged octagon style. Also the repurposing of such a barrel by adding the powder pan would mean it was still ussuable and not too far worn down. What puzzles me is that the barrel must have always had the touch hole on top , which at that time was not ussual (the touch hole moved to the right hand side from about 1450s to its final place around 1490/1500).
The second example shows a very similar tiller cannon to mine with the barrel mouth flaring in a similare way and a complete powder pan with swiveling powderpan lid (the powderpan lid and screw now missing)