Al parecer, esta marca puede ser debida al proceso de impresión de los billetes, cuando una parte con mucha tinta por un lado coincide con otra muy clara en el lado opuesto. Ya que al acumularse las diferentes capas de billetes y rozar un billete sobre otro esto hace que la tinta de un billete manche la del billete que está encima.

"My last experience in the printing business was in the early 1950s, so it may not be relevant in today's industry, but your quoted comment might contain part of the answer to your question. Intaglio printing is almost like embossing, resulting in high and low areas. If there is a high area that is rubbing against the heavily inked opposite side of the adjacent sheet of bills, there could be a slight ruboff causing that mark. The ink used on U.S. bills does have a tendency to smear a little when rubbed, and in the printing process each printed sheet rubs against the previously printed sheet." ,

ROY Satootoko, from the Coin talk Forum.