To attempt to address your question, Paul—I found something about the current state of 1-15 device polarization—sorry this is not a historic perspective, but does explain what has evolved a bit.
See:
http://67.115.161.42/imag/ecn.nemawd.1-15R+P.jpg for some nema dimensioning. The 1-15R drawing calls out for a female polarized receptacle only, but there are two 1-15P male cord caps—one polarized and one non-polarized version. My guess is that for ungrounded, maybe double-insulated or unswitched appliances, there is accommodation for that in design flexibility.
