Performing some routine maintenance on a used '99 I purchased, I decided to check out the thermostat. WOW, I'm glad I did, looking back I found a piece of metal in the radiator but did not make the connection until I saw this. It looks to have been like this for some time as the spring has worn a groove into the brass cylinder.
I am going to place it in boiling water to see if it still works, but I don't believe it matters as water was still going past the broken outer ring.
You all have likely seen this before but it's new to me, and no, I'm not starting a do vs. don't run a thermostat thread!