Dri-Mark 351R1 Smart Money Counterfeit Bill Detector Pen for Use w/U.S. Currency (Pack of 12) The original and best Counterfeit Detector Pen, patented by Dri Mark in 1991. Superior chemical test for counterfeit US paper money. A simple mark on paper currency: dark mark means counterfeit, light mark passes test The only Counterfeit Detector Pen still made in the USA Superior quality means superior performance