I'm just about to put the cooling part of the engine back together and I was going to check the thermostat. I have an external thermostat on an '81 FI engine. It was cooling fine before so I just wanted to clean it and ensure it is opening at the right temperature. After cleaning I put my finger in the thermostat housing and noticed a loose thin rubber/gasket piece or something (I can't see it). Can the housing be unscrewed to examine or is it welded shut?

The thermostat seems to open fine and holds water when closed so need I worry?