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.*
*Wikipedia
You are looking for a metal rations tin please replace as found.
To Open Cache undo buckle and then loosen the strap towards the back of the cache and lift lid straight up.
In the cache attached to the log book is a self inking stamp THIS STAMP IS NOT TO BE TAKEN FROM THE CACHE please let me know if out of ink and I will do my best to refill it but if you are after the stamp it might be as well to bring your own inking pad. The stamp works best on totally flat surfaces.
Well done to Megamax for 
FTF