United States. Department of the Treasury. "Bill Tender, May 20, 1968, Release Number f-1255," in United States. Department of the Treasury. "Volume 161," Press Releases of the United States Department of the Treasury (March 4, 1968 - June 3, 1968) : 589-590. https://fraser.stlouisfed.org/title/6111/item/587004/toc/541059 , accessed on June 24, 2024.