FYI I often find ether ruminants of red paint or pronounced red paint mark on the thermostatic spring on many units I have seen or had. I have seen this on assemblyline units as well as some replacements. It appeared to be a quick spray shot that was confined to that small area because of the minimal overspray. I am not sure of the meaning but just that it was some what typical from what I have found.
That's interesting, I have seen some red marks/spray on some of my Eaton Corvette clutches from the '60s and early '70s. Here are some pictures, NOS in the middle, and the last one is a well used old one that when I cleaned the dirt/junk off the spring it was reddish on one side.