The early 100-6 cars, as described in the link above, use a different temp gauge fitting at the sending unit than do the later 100-6 and 3000 cars. The early 100-6 cars use a temp gauge sending unit that connects to the radiator. That sending unit uses a female threaded nut to make the connection. The later sending units use a male thread that goes into the cylinder head. To use the early sending unit and to fit it into the head, instead of into the radiator, does require an adapter. If using a later sending unit, it's a different story, with likely no adapter needed.