This World War II propaganda poster encouraged Americans to buy war savings stamps. These were issued in small denominations that could be collected and exchanged for war bonds. War savings stamps were issued during World War I and World War II. According to Wikipedia,
The war savings stamps introduced during World War II were released in five different denominations – 10 cents, 25 cents, 50 cents, one dollar, and five dollars, all featuring a Minuteman statue. These stamps were purchased at face value and earned no interest. Individuals accumulated their war savings stamps in various collection booklets provided with the purchase of a stamp. Filled collection booklets could later be used to purchase Series E war bonds. For example, a full 25-cent booklet contained 75 stamps and was worth $18.75, which was the initial price of a $25 war bond. Thus, a full 25-cent booklet would be exchanged for a $25 war bond with a time to maturity of ten years.