This is a tiny geocache container with a squared hole for a 20mm x 20mm magnet placed in the base. You need to pause the print in order to place the magnet before the nozzle covers it with plastic. It has a screw cap and it can contain a small log. 

On the surface, a shallow indentation allows you to apply a tag 37mm x12mm like this example:

You can place this magnetic cache where some iron is present, like in the structure of a bench, bridge or pole.