My thermostats are stuck open and I was wanting to replace them. While researching parts and pulling my old stats I see differences. Mine are what I call "open" bodied thermostats, but the diagrams and parts pictures for the ones listed for my motor look like they should be in a plastic housing. Which should I order? I come up with a part number of 435491. Any of you run across this or have advice? And why the two designs?