Feltwad,
I think you're on the right track.
To me, it's an old fowling gun or bumper, made for shooting at birds on the water.
Hudson lock could be Thomas Hudson, (1) as it seems to fit time-wise, and this lock is likely before 1765.
I think the barrel may be older than the lock, maybe a good bit older. If made in sections it could be Spanish or as you say Italian.
Don't think it's a trade gun, and think the old way of fitting the tang screw upwards is because the barrel was made when this was still in vogue. (say late 17th C.)
I also believe it May have been re-stocked in it's working life.....very plain and most likely a provincial job.
A lot of carpenters used to get jobs of this type....plain working pieces needing repairs or a new stock.
We see " newer" working guns made very plainly as well. The well known photo of Snowdon Slights with his punt and duck guns shows most of these to be the sort used at Fisherman /fowlers, who made their livelihood shooting and fishing. Mind, his old percussions look like they started off as flint, so may be as old as this one.
In all, a decent working gun for wildfowl I'd guess.
PS, another reason to think re-stocked/newer lock or whatever, , is that the lock is a lot better quality I'd say than the t/guard and stock.
Richard.