Hey, something else I just found while researching
this thread. 1951 NEC, Sec. 2111(1): Receptacles connected to circuits of more than 150 volts between conductors shall be of such design that attachment-plug caps used on circuits of other voltages on the same premises cannot be inserted into them.
1953 NEC, Sec. 2123(b): Grounding receptacles installed on circuits of less than 150 volts between conductors shall be approved for use only on potentials less than 150 volts.
(italics mine)
I guess that's the origin of the divide between parallel and tandem right there.
Only for grounding receptacles. The 1951 language leaves plenty of wiggle room (e.g. you could always use a crowfoot or a twist-lock for 240V), but the 1953 NEC effectively eliminated the 5.5-15R. (Though not the non-grounding T-slot!)