I have no clue as the purpose of the plug but there's a long unused synapse firing that is telling me the circle with the indents next to the plug was a line code identifying some aspect of the casting process. Possibly a date "stamp" or batch identifier? I could be completely wrong but that synapse won't shut up. You see there's another in your second picture.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.