Know these adaptors.
Still think he found it easier just to kill the ground prong.
BTW: are all the ground prongs on NEMA 5-15 plugs hollow or are there also solid types?
We were on a class trip back then, and a classmate told me at his place all the receptacles missed the covers (and later some curtain fell down on his head with all the hardware).
This is West End Avenue, Brooklyn, if you know that area. Just some minutes to walk from coney island beach, close to the D train.