Are you talking about those 3 ports?
View attachment 2823470
Better view on the 2 rear:
View on the center one and the first of the rear:
View attachment 2823486
My head has partnumber 60034, it's from a euro-spec F.5 built in 10/1973.
I have no idea what is supposed to be the usage of the center one, I only discovered it following the restoration. It's very hard to see it once the engine is in running condition, you have a very narrow angle to see it between the carb, airbox and valves cover, and it's always in the shadow.
My euro 74 so has the 3 ports on the head but also has this thermo valve on the thermostat housing. It's on the way of the vacuum advance but not too sure how/when it supposed to work (and even less if it's still working).I believe they moved the emissions thermal valves to the thermostat housing, possibly for more consistent readings, or, it's easier and cheaper to drill and tap an aluminum casting than the cast iron head.
My US 2/78 head has three ports. Stamped 61031. Never could figure out why the extra port was there as I've got multiple thermo switches on the thermostat housing.