I've recently snagged up a ~1949 239 with 8ba heads. This motor is stuck and in severe need of a rebuild so first step was to tear it down however when I came to the first head (did the right side one first) I thought I took all the head bolts out but apparently not when I realized I couldn't get the head off. Quick moment of pondering and I realized that a large sensor was in place of a head bolt and that it must be holding it down. Sure enough I loosen the sensor and an attached stud comes out with it.

​​​​​​The sensor is a good 9 inches long end-to-end with a brass(?) tip, section of unthreaded shank, and at the top is a head stud with a nut that you would presumably tighten to spec topped by a ~1 inch diameter bakelite sensor with two spade connectors coming out the top...
Both heads had one of these sensor-studs and I'm curious as to what the h-e-double-hell is it? I couldn't find anything online for temperature sensor studs (the motor had the usual two other sensors on it as well) and I couldn't find anything relating to an oil temperature sensor.
Vanpelt sales diagrams came up dry as well as I didn't see any other flatties with that type of sensor.

I'll attach a picture of it later tonight.

Any ideas are appreciated.

 

Would it be an engine block heater, like the one below?

 

Could be a block water heater.