It's a pain in the butt. Recalibration means physically trimming the crystal. Which side you trim depends on whether it's too fast or too slow. It's really fidd

It's a pain in the butt. Recalibration means physically trimming the crystal. Which side you trim depends on whether it's too fast or too slow. It's really fiddly though (especially too slow, which requires grinding the ends down instead of the face). Even in the 20s they would have been producing these dozens at a time with a big lapping wheel, so they could just pick a better crystal from the same batch.