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The problem was that ranges and dryers began evolving many 120vac assessories, lights, timers, bells, whistles , etc....
This meant current was 'returning' on the grounding conductor. Granted, a negligible qantity at first, but increasingly so, thus this introducing a hazard.
So it became a code ( i'm thinking the 93' or 96' cycle without looking) to have a 4-prong receptacle and 'pigtail'
( cord from unit).
so now we have Hot-Hot-Nuetral-Ground
This is not so far fetched if one considers the modern day receptacle that has Hot-Nuetral-Ground ( looks like a face with the ground down)