This is a tool for setting the correct focal lenght of a K40 laser cutter. The K40 has a focal length of 50,8mm. For the best cut the focus center should be in the middle of your material, so a material of 3mm thickness the focal length is 49,3mm.
My tool has the lengths of 46,8mm 47,8mm 48,8mm 49,3mm, 49,8mm 50,3mm and 50,8mm.
You can read more about here: https://k40laser.se/diy-how-to/setting-the-proper-focus-in-your-k40/