I was wondering how well counterfeit pens work on old bills. I just got, in my change, a 1950A series $20 bill. When I tried to spend it, the cashier took out her counterfeit pen, and marked it. The Counterfeit Pen said that it was counterfeit. I took the bill back and gave her a different one. My question is, is the bill fake? I did some research, and apparently the pens use an Iodine solution, that reacts with starch. The changed bill composition in 1960. Is my bill real, but the cashier did not have advanced enough technology, or was she right, and my bill is fake?