I've made some postings on the NSSA board about this item, if you are interested. It has turned out to be an interesting research project because if you read the popular books on Civil War and pre-Civil War U.S. artillery, you might get the impression that the US did not buy any bronze 6-pounders between 1801 and 1836. It seems that was not the case. "Brass gun" was the common name for guns made from gunmetal bronze, an alloy of about 90% copper and 10% tin.
http://www.n-ssa.net/phpbb/viewtopic.php?p=30306#30306
http://www.n-ssa.net/phpbb/viewtopic.php?p=30306#30306