The United State's heavy involvement in the World War I cost around 30 billion dollars, hence to raise funds in the World War II, the United States Treasury Department issued War Saving Certificate Stamps. These stamps were different than postal saving stamps; US Post office played a major role in distributing these certificate stamps.
Unlike the liberty bonds of that time, which were purchased by the financial institution, these war saving certificate stamps were primarily aimed at the commoners. The lowest denomination issued by the treasury was the 10 cent stamps. The first war saving stamps was issued in 1917, to help with the war efforts. The collection of war saving stamps could be redeemed for treasury Certificates or War Bonds.
The last series of this War Saving Certificate Stamp was issued in 1920 and its maturing date was in January 1926. These stamps were affixed in an engraved folder called War Saving Certificate. The owner of these stamps can redeem the interest of these stamps prior to maturity in cash.
Image courtesy: http://stampauctionnetwork.com