Thank you! The thing in the bearing is a nut with two different threads on each end. It's called a "reducer" here in the US. I wish I could say that I made them myself but I bought a bag from China...I now have a good handful I don't need
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3/8 16 on one side (Roman Emitter threading) and 1/4 20 on the other (good size for real sink knob hold)
Bathtub tap... where everything is correct, but the square hole is bigger in the metal piece and which also has deeper threads so that it isn't possible to flip the cubes...
...which is a variant that I have with one of my tap handles....