Pretty sure that port is where the thermocouple end goes that comes from the temperature gauge. It tells the dash gauge what the engine temp is. The thermocouple is a small sealed hollow tube that is filled with gas. If it is pinched or cracked, it will not work. The coolant block shown coming off the engine has three ports. One for the thermo-time switch, one for the coolant feed to the WRD and one port for the thermocouple.
I am surprised coolant didn't spray out of it immediately after the engine was started?