Well I can say I really lucked out again.
The wildcard piece was certainly something I was stressing over but when I opened the package and unraveled the bubble wrap seeing this gorgeous piece, I was giddy. I then realized I'm possibly the absolutely lone person to get this excited about some really old and dirty dishwasher thermostats.
I've never seen these with a rubber gasket before in all my research so this is something new to me. It quite certainly suggests that the thermostat is installed inside the main compartment of the washer that gets filled with water. And yes, this rubber gasket is kind of gross. I will be cleaning both up. Fortunately, the original one I feared had corrosion indeed does not. Just a layer of crud that needs to be scrubbed off. It appears to possibly be some kind of limescale, so I think a vinegar bath will do the trick.
I am now thinking once I clean them up and get my one loaner back, I can mold all 4 at once together.