I just made a few of these little dandys.
They are shims that are placed within the split bolt of the Ruger 77/22 to adjust the chamber headspace. A 22LR firearm should be headspaced to the absolute minimum to get the most accuracy possible from the gun. Many guns will come with far too much headspace and associated problems because it is cheap and easy to manufacture that way.
Machining shims this thin involves some machining tricks that most machinists have learned over the years. Cool stuff but time consuming.
The SAAMI specification for the 22LR round and chamber. Note that the headspace for the chamber should be between 0.043″ and 0.051″.
Forster headspace guages. Go (0.043″), NoGo (0.046″), and Feild (0.049″). These are the only production guages available. I could get some custom cut if I end up doing a lot of 22LR projects.