You need to separate the two pieces of the sensor bulb in the last photo. Then the square head fitting will thread directly into the head. The sensor bulb has a shoulder that bottoms out in the counterbore below the threads in the head. That adaptor appears to have a 1/2" NPT male thread to fit the later model heads.
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!