Apparently from a solider in I Company, which was the letter name of the 13th’s Minutemen of Attala, though I have no idea whether the buttons are from one of them.
UPDATE: Winston Cameron of Winchester, Virginia, a descendant of Captain Hugh Cameron and First Lieutenant John Cameron, both of the Alamutcha Infantry, thinks the I on the buttons more likely stood for Infantry.
13TH MISSISSIPPI INFANTRY REGIMENT 
Reblogged this on Poore Boys In Gray.
The “I” does stand for infantry. There are also Mississippi buttons with “A” and “C” on them for artillery and cavalry, respectively. I’m a Civil War relic hunter, but I’ve never found a Mississippi button. Those are very rare and very beautiful pieces to have in a collection.