I haven't got the part removed from mine, but you can see it on the one in this ebay auction:
https://www.ebay.com/itm/371818637727
The port in question is the little small part above the temperature sender threaded port. When mounted on the engine, it is at the highest point in that thermostat housing and has a hose that connects with the vent line from the top of the radiator to the expansion tank.