A series of late 19th century USA stamps are known for having “grills”. What is a stamp grill? Basically, a grill was a security method developed to prevent the fraudulent reuse of postage stamps.
Every time a stamp is used it is cancelled at the post office. A stamp can be cancelled by hand with a pen, or with a specially made canceling device. Unfortunately, many people would attempt (successfully!) to wash the cancellation ink off the stamp and then reuse it instead of buying a new one.