It is a .58-caliber "six-arm swaged" yankee minie bullet. There is no .54 version of that bullet. The actual correct terminology is "Machine Pressed & Turned" which means a bullet-making machine compressed a slug of lead into a bullet shape, and the machine then lathe-cut the body grooves into the bullet's exterior. Yes, a little of your bullet's original length has been carved off of its base. No such bullets were manufactured with a letter T stamped into the base, so what you are seeing must be an accidental result of the swaging/lathing process, or was done by the person who carved on your bullet..