Fascinatin': The story has to be referring to currency in a way I've never heard, and I've read piles of archaic literature. Either the implication is that there was once such a thing as a 5-cent paper bill, or "Billy" is referring to a "stamped" coin as a "stamp". If a metal nickel is being referred to as a "toadskin", I wonder if that usage has ever existed outside the particular author's imagination? That expression for paper money is much easier to understand.