This is probably the tiniest letterbox you will see. This will be a fun find. Promise. Bring your own stamp and ink. The stamp for this letterbox will remain with the cache.
Tribute to letterboxing
Letterbox Hybrids are based on an older kind of container search, called letterboxing. Because letterboxing began in 1854, before GPS existed, the finder follows written instructions to discover the container.
Each letterbox contains a logbook, and a rubber stamp. When letterboxers find the container, they stamp the logbook with their personal stamp, and also stamp their own notebook with the stamp from the letterbox as a souvenir of their visit. The stamp and logbook remain in the letterbox for the next visitor to use.