Apparently, this thermostat design is only related to the Series II with Strombergs. Since I fitted the correct thermostat with full opening at 74 deg C my engine temps via the dashboard gauge have been very consistent. Cross referencing the upper and lower hose temperatures with the temperature gauge reading has given me confidence about my engine temperatures.
Thank you for discussing your results. I will make sure that mine has the correct thermostat when I get to that point.
Apparently, this thermostat design is only related to the Series II with Strombergs. Since I fitted the correct thermostat with full opening at 74 deg C my engine temps via the dashboard gauge have been very consistent. Cross referencing the upper and lower hose temperatures with the temperature gauge reading has given me confidence about my engine temperatures.