In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
It's under the enclosure on a 1910 convector in the dining room. The wires disappear through the floor into the sheetrocked ceiling below and there's nothing in the boiler room to indicate its presence.
Some kind of temperature limiting device when the switch is in the "on" position? When it reaches setpoint, it closes a valve?
Often wrong, never in doubt.