The gun is mounted on a naval slide carriage, so it is not a field gun - more like a small deck gun. It was also provided with many dozens of solid spherical shot, apparently every New Year for many years one or two were blasted into the Bay by the later family there. There is a historical re-enactment society in Victoria, who re-enact only that ship's crew, they have, apparently & understandably, done some research re this gun & dispute that it was from the CSS Shenandoah saying she was not armed with such a gun. The Shenandoah was a very successful raider & did a lot of damage - it is not difficult to imagine that she would have relieved at least one of her armed victims of an appealing gun or two - and finding them surplus to requirements later (probably quite a nuisance) & giving them away - getting a small gun like that unloaded would be a trifling for such a ship.
Her story is an interesting one, after the war it cost the British Govt, six million dollars, a vast sum, to compensate the United States for damage she caused after leaving Melbourne - as a neutral colony the ship should not have received any assistance etc in Melbourne. Although Melbourne's citizens were divided on the ship being in Port the captain & officers were feted & looked after by many elements of Melbourne's high society &, apparently, the cannon was given to Mr. Samuel Ames, a member of the City Council (eventually Mayor) in gratitude.
Adrian