I don't know if you saw my post to Greetings From Boston Man, but I found an article in the Southwestern Historical Quarterly July 1935 issue written by Lt. Cmdr. David P. Marvin, USCG, ret'd., a faculty member at the Coast Guard Academy. In that article he says:
Lying at the rendezvous off the bar, Captain Faunce observed a steamer approaching, and as she had no colors flying, the Lane fired a 32-pound shot from the bow gun, as an order to halt. This shot, at 11:20 a. m., April 12, 1861, was the first of the conflict to be fired from the deck of a Federal vessel.The stranger proved to be a merchant steamer from New York, the Nashville, with passengers and freight, and was allowed to pro- ceed to Charleston; she was later taken over by the Confederate government, used as a cruiser, then as a ram, and was lost in battle.
I did find records where the Lane had exchanged its Revenue Cutter flag for a Navy flag, so perhaps the various paintings of this shot that show the Revenue Cutter flag are incorrect. See Flag link, see lower left.