I had some initial confusion as well until I went to Mansfield. Different books say differnt things about the 19th's inclusion.

On the marker at Mansfield, the 19th is listed toward the bottom as being part of the army, but not at the battle.

Just FYI, the Mansfield battlefield is very endangered. There is a strip mine that is encroaching on the battlefield site, and the state of Lousiana has not stopped it.