According to patent
"Laser adjusted set-point of bimetallic thermal disc"
it is done at production , for precise adjustment, but no specific details
of patterns/which surface/degree of variation. I'm more interested in
adjusting otherwise useless batch made for set point of 16 degree C up to
50 or 60 degree C, with no great precision, +/- 10 deg C would be fine

If you could find the patent for the controller I described (it's a famous
patent, from a man who patented many devices in the 50s and 60s), it might
show how this is done.