I think you folks are correct. To make it slightly more complicated, I remember them being referred to as "frost plugs". And the internet says "freeze plugs". Since the replacement "core heater" such as I added to my Toyota installed with an expanding rubber gasket, I presume that it can serve the same function in addition to its heating function, should the hot part not be plugged in or the vehicle abandoned out in the cold.

In the example I mentioned above in my previous post, a lot of persistence plus some luck allowed me to get out of that situation without calling for a tow or a more intelligent mechanic.