This is an unusual variety caused when the engraver mistakenly punched a 0 into the die where the D (of DOLLAR) was supposed to go. Rather thad discard the die and start anew, the engraver simply punched a D over the 0. Actually, this error occured in 1818, then the die was reused in 1819. Based on the dramatic shift of the 5 to the left, it is possible the engraver was confused and thought he was working on a Half Dollar die, but this is purely speculation.
The 1819 $5 5D/50 is a rare variety -- Dannreuther wrote that less than 20 were known. The finest example is a single PCGS MS65 from the Garrett Collection.