I see where a sensor could be installed (exactly where it should be according to the service manual and other threads) but it looks like the hole is sealed off with a stud- looks factory to me. I did find what looks like a sensor on the passenger side at the same location. It has a single purple wire with a 90 degree rubber boot- you can see the small black rubber boot just above the grey front spark plug boot in this photo. Could they have just put it on the opposite side or is this some other sensor?
Hopefully a Gen I person will chime in. That other post I linked with the broken off sensor(switch) talks about using the same hole in the passenger side head as an alternative location for the sensor. HOWEVER, the sensor hole on the passenger side should be toward the back of the engine. The two heads are the same part number, so they should be built with the hole machined in the same location., i.e., just above the 1st spark plug hole to the far left when you are looking at the exhaust side of the head. So, that purple wire in your last photo doesn't seem to me to be related.
I looked up the part number for the thing you are looking for. It is J3230408 and is called "SWITCH, Engine Temp. Warning Lamp" in the parts list, and looks like this (from eBay seller of an OEM part). Is that possibly the "stud" that you mention seeing on the driver's side head?
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