The early cylinder heads did not use an adapter. The thread on the capillary tube nut should fit the head without any adapter needed. The 1/2" NPT sensor thread came later, and needed the adapter. There should be something like a 5/8" fine thread in the head, and a flat seat area for the capillary tube bulb to seal against.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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