Got enough energy left for a post tonight. The "3-ringer" (actually, 3 grooves, not rings) Minie-bullet you found is the US Regulation .58 Minie, manufactured by Elijah D. Williams. It first starts showing up on civil war battlefields in 1863, where the Army of the Potomac was fighting in Virginia and at Gettysburg. It got used in other states in 1864 and '65. See bullet #156 in the "Handbook Of Civil War Bullets & Cartridges.".