On the later heads the hole in the head is just a straight pipe thread - - - -there's no seat area for the sender. The adapter shrinks the hole to the right size and thread and provides a seat for the sensing bulb.
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!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!