SOME HISTORY:
The origin of letterboxing can be traced to Dartmoor, Devon, England in 1854. William Crossing in his Guide to Dartmoor states that a well known Dartmoor guide (James Perrott) placed a bottle for visiting cards at Cranmere Pool on the northern moor in 1854. From this hikers on the moors began to leave a letter or postcard inside a box along the trail (sometimes addressed to themselves, sometimes a friend or relative)-- hence the name "letterboxing". The next person to discover the site would collect the postcards and post them. In 1938 a plaque and letterbox in Crossing's memory were placed at Duck's Pool on southern Dartmoor.
The first Dartmoor letterboxes were so remote and well-hidden that only the most determined walkers would find them, allowing weeks to pass before the letter made its way home. Until the 1970s there were no more than a dozen such sites around the moor, usually in the most inaccessible locations. Increasingly, however, letterboxes have been located in relatively accessible sites and today there are thousands of letterboxes, many within easy walking distance of the road. As a result, the tradition of leaving a letter or postcard in the box has been forgotten. (Wikipedia)