what? its even easier to trace money without modern tech. The coinage wont be carbon copies of each other. They're less robust and easier to mark, there will be differences in coinage between cities, kingdoms and regions. Their (swindler and mc) will have had different travels and accumulated different coins.
Aside from looking at the currency as a whole, there just needs to be one unique distinct feature that someone else can confirm in the bunch. Like a coworker giving her a unique coin.