The Safecracker 40 is based on a design from 1911.
The aim is to align all 16 columns so they each add up to 40 at the same time.
There is a base layer with a further four rings on top which rotate. There are a lot of possible combinations for this to sit at but only one solution.
This is made from alder with the numbers laser engraved and measures 110mm in diameter.
The solution is included.