I hope this question is inside the scope of the forum. I have been wondering why some US electrical plugs are polarized (one blade larger than the other, so as to fit only one way in the socket), ...

General Electric polarized electrical adapter with ground tab. ca. 1970. An adapter to connect a 3-prong plug into a two-prong outlet. Molded on front: "GE / UL / 15A 125V", on back: "Caution / ...