Think how dies are made. The die, to make such, would have been a hand die. Or dropped or misplaced. Today, they do not use hand die, right.
It is created by grease, dust and debris being compacted into the incuse, on the die, T of Liberty. Coins struck prior to this example would have a missing T in Liberty, and known as a struck through grease error, (minor). This compacted debris then falls out just before the next planchet is struck and can land just about anywhere. The planchet is struck and you have the OP piece which is known as struck through a dropped element.
From the size of the dropped letter I would say it was one of the T's in TRUST. T from LIBERTY would be much larger.