Actually the item in that photograph is a "Limber" not a caisson. Also, since both northern and southern forces used the same specifications, a Limber could have been manufactured in the north and acquired by southern forces during or after a battle. But since it is labeled as living history, it was likely reproduced from the circa 1850s "Mordecai" drawings and specifications produced under the supervision of Captain Mordecai as commissioned by the U.S. Army.
http://en.wikipedia.org/wiki/Limbers_and_caissons