Most people probably think "stamp collecting" involves postage stamps exclusively. But there are other types of stamps in philately such as Revenue Stamps and, a particular interest of mine, Savings Stamps. The latter is related to currency and Scripophily (stock/bond collecting). This is probably the best such item I've come across, a block of four of the 1918 $5 War Savings. The majority of this issue (Scott # WS2) has round hole perforations (11 per 20mm). This is the much rarer (Scott # WS3) version that has rouletted (slit) perforations (7 per 20mm).
The printing on the stamp is self-explanatory. The purpose was to function similar to a savings bond. I believe that this item is still valid for $20 (but is worth more than that as a collector item, of course). This example is in excellent condition and the original gum (glue) on the back is undisturbed and was never stuck down to anything. Most of these types of stamps were stuck down in one of several types of folders available from the Post Office (or later affixed in a stamp collector's album). The owner would cash in the folder when a certain total dollar amount and date was reached. The stamps would then be cancelled and generally retained by the Post Office.
These particular stamps are quite large in size. This shows the size of a block of 4 compared to a small-size US currency note: