That eBay plug appears to be the larger size for the front or rear of the head, not the top.It's the same hole. The early cars just had a plug there, the Alfetta and L-Jet spiders got a temperature sensor.
Joe, I got one on eBay. There is one there right now:
Alfa Romeo 2.0 4 CYL REAR CYLINDER HEAD PLUG Spider GTV Berlina Alfetta engine | eBay