""Savings Stamps" is a general category that includes 4 slightly different types of stamps issued at different times by either the US Post Office Department or the US Treasury Dept.
Their common feature was that they all effectively acted as a means for ordinary citizens to save and/or invest incrementally, a little bit at a time.
At the same time, the investments effectively were loans to the federal government.
The 4 types are Postal Savings Stamps (issued from 1911-41) and Savings Stamps (1954-1961),
both issued by the POD; and War Savings Stamps (1917-45) and a Treasury Savings Stamp (1920), both issued by the Treasury Dept.
Postal Savings Stamps were issued in 10¢ to $5.00 denominations and were redeemable in the form of credits to postal savings accounts. The 1911 10¢ stamps were available either as stamps or as a stamp imprint on a card, to which other stamps could be added.
The postal savings system was discontinued March 28, 1966.

Savings Stamps, also issued in 10¢ to $5.00 denominations, are the savings stamps most familiar to older philatelists. These collectors may remember buying these stamps at school and placing them in booklets. When full, the booklets were redeemable in the form of US savings bonds. Sale of Savings Stamps was discontinued June 30, 1970.
War Savings Stamps were issued in 25¢ and $5 denominations (1917-20) and 10¢ to $5 denominations (1942-45).
They were redeemable in the form of US treasury war certificates, defense bonds or war bonds.
One Treasury Savings Stamp with a $1.00 denomination was issued in 1920. These stamps were redeemable in the form of either war savings stamps or treasury savings certificates"