The screw plug in the top of thermostat serves 2 points , one, to take a sensor , 2 to help bleed the system, on my 103 it has both one on thermostat and also one on the block rail and i use them both to purge the air .I must say it is hard to get all the air from the system. Just about to do this job again as the coolant needs changing and i am not looking forward to doing so .