135° safety thermostat with an M4 stud. These are resettable, but you need a pin to get in the small hole in the center.

Only found in older, out-of-production models (such as the "Roma"), with the exception of the current "You" model. 

EXTREME care should be taken when installing this part, as the stud has the potential to break off inside the boiler if over-tightened. It may or may not thread all the way in flush to the boiler, and it doesn't need to: slightly over hand-tight will be sufficient.

aids heat transfer between the switch and contact surface by filling in microscopic imperfections in the metal surfaces. It is especially useful for pipe mount applications.

A key characteristic of this synthetic thermal grease is it's long-term material stability. That means it won't harden, dry out, or melt in normal industrial use.

Smear some paste on the sensing head of the thermostat switch prior to bolting or clamping down. (Not to be used as a glue.)