Part identification: Thermal cutoff within baseboard heater
Possible manufacturer could be: https://www.emerson.com/en-us/catalog/therm-o-disc-10h

Product bulletin describes how the thermostat works:


  The 10H capillary tube is vacuum-charged with selected fluids to give specific calibrations. When the calibration temperature is reached, a change in fluid vapor pressure allows the diaphragm to snap through and operate the contacts. 


It might be impossible to source just one of those over-temperature switches from them. I'd look to replace the whole heater if an original or an approved replacement for a safety critical component can't be found.