Here we have a wooden block with a locked top and various numbers branded on the back & front.
The finder will be given the hint that a coin or car key might be useful and this will provide the digits required to open the lock.
With ferrous object in hand the finder can check each number in turn to discover four of them will attract the object and thus provide the code required for the lock.
A waterproof capsule & logbook is provided.
Youtube Clip - https://youtu.be/NUjIENfPyhU