Antique thermostat

[Jack Guzman]

I am working on the cooling system for my 51 3100. The 235 in it is a replacement. No idea what year. I removed the thermostat to inspect it and was surprised with an unfamiliar object. It is obviously a thermostat but instead of the familiar cylinder with a spring coiled around it it consists of a bellows as big around as the opening it rides in.
What I'd like to know is how long ago this style was replaced with the modern version.---Jack