Hi Fernando, 
                My knowledge of older firearms is limited but I am of the opinion that this must be a signaling mortar of some sort. A cannon that would be used to fire a projectile in any way effectively would have a much smaller diameter touchhole, even taking into account wear and tear and corrosion over the years, and probably a longer barrel to bore ratio. The size of the touchhole on this example would be more suited to, as you say, a fuse e.g. hemp impregnated with saltpetre or a thin tube filled with black powder. Hope this is of some assistance. 
                       Regards, 
                       Norman.
		 
		
		
		
		
		
		
		
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