These can still be bought today.  We put one on a 1933 International Pickup that we restored.  It is sort of akin to a thermostat and you can tell if things are getting too hot.  Your image shows a fancier one that we bought.  I don't remember if we got it from the antique cars parts supply in OKCity or ordered it from the Hemmings Magazine.  But they were quite common in that era.