What a mess—I meant to say “the 1971 NEC, not 1975”.
But reading it more carefully it looks like the thread did conclude it was 1975. Had some trouble following it!
And making things more confusing, Wikipedia says 1971, but the source they cite doesn’t actually back up that claim. See https://en.wikipedia.org/wiki/Cheater_plug
What a mess—I meant to say “the 1971 NEC, not 1975”.
But reading it more carefully it looks like the thread did conclude it was 1975. Had some trouble following it!
And making things more confusing, Wikipedia says 1971, but the source they cite doesn’t actually back up that claim. See https://en.wikipedia.org/wiki/Cheater_plug