Check out this picture. This water heater has a temperature limiting device that shuts of power to the element so the valve at the top would only need to be a pressure relief valve and a discharge pipe. Obviously there is no discharge pipe but possibly the round part at the top could be a knob for a PRV. I have never seen one like this before. My first thought was that the valve was missing altogether but there was pressure on the system so something is keeping the pressure in. Any of you recognize this?