@dlacey
Wes Ingram's shop reconditions WURs. AFAIK, the thermostat is NLA, so a rebuild is usually just dismantling, cleaning and here's the most significant part...replacing the rubber seal between the upper air chamber and the lower water chamber.
I've seen that seal removed and it's just a think rubber washer. Hard to see in your picture, but I've seen the mangled, deteriorated remains on the work bench. The replacement being a handmade version cut from a sheet of neoprene. Once the old seal is removed the thermostat falls out.
That's all I've got. Hope it helps. Nice pics and illustration BTW.