Thanks. As far as what craftsman decorated it, we'll probably never know, but the work was ordered by the Master General of Ordnance (Mulgrave?) so the cannon could be presented to the officer's father, also an officer in the RHA. The officer who used this previously-captured piece at the Battle if Nieville (sp?) in 1813 was Lt. William Livingston Robe, RHA, who was killed at the Battle of Waterloo in 1815. The cannon is essentially a memorial to him.
There's been extensive discussion of this piece on the "Napoleon Series" forum, trying to find the exact specs mentioned in the text of one old manual or another. I've been following those topics and as far as I can tell, nothing even close has been found in the old Portuguese museum catalogs, and nothing much like it was produced in France before 1813. There was one French mountain gun included in the "System XI" but Napoleon I suppressed it before 1810, so I don't know what it looked like nor how many were cast, nor if any remained after that model was suppressed.
So every indication I have is that this was indeed a Spanish gun (it has no chamber.) It is very close externally to the Spanish Navy's 3-pounder howitzer, which of course had a chamber. From all of the old literature we can find, the chambered pieces, both 3 and 4 pounder, were all originally cast to Spanish Navy drawings. We've found Spanish Army drawings and specs for unchambered 4-pounder mountain guns with the same basic exterior form as the Navy pieces, but we have yet to find Spanish army drawings or specifications for an unchambered 3-pounder gun of this form.
Why do we want to find the specs and drawings when we have photos, measurements etc, on the 3-pounder unchambered "trophy gun" already? It would be nice to know the exact name the Spanish used for it at the time. When that is discovered, anyone can then search Google Books using the proper terminology and probably find out a lot more about them.
More research is required.