1965 413 thermostat blew up

My New Yorker temp started to creep up yesterday. I let it cool down 1/2 hour , and when i started it, it wouldn't go down.

Tore it apart and found this? What happened? I had posted about this thermostat a year ago, and was told " keep it, best thermostat you'll ever have"

What happened?

View attachment 466848
Author: mopar440