United States. Department of the Treasury. "U.S. Savings Bonds Issued and Redemmed through May 31, 1967, June 6, 1967," in United States. Department of the Treasury. "Volume 158," Press Releases of the United States Department of the Treasury (June 1, 1967 - August 29, 1967) : 20-21. https://fraser.stlouisfed.org/title/6111/item/587001/toc/540717 , accessed on May 19, 2024.