Well I have a good one for you.
I bought the truck in 7/14. It may have been then or a year or so later, I bought a new Motorcraft Thermostat from the Ford dealer in Santa Cruz. I had it for a couple years, then one winter the Water Pump started leaking, so took it to a shop because I didn`t want to work out in the snowy cold.
I gave them the MC Thermostat to install.
Everything was working alright. Some time later the water temp would not heat up.
I pulled out the Thermostat, and in the center was this piece of rubber keeping it from closing. Pulled it out, and (now I don`t remember it I reinstalled it, or replaced it with another MC)
When I was filling the radiator back up, up floats the rest of the rubber piece.
So far it all works ok and I have heat, and this is probably 5 years later.
I don`t know what the rubber piece does or is for. As mentioned there is the gasket.
Goat