Found another strange outlet, this one made by Bryant. Rated 125V 15A or 250V 10A. you could get a NEMA 5-20 or 6-20 plug to fit it if you broke off the grounding pin. A NEMA 2-20 2 pin 20A plug is a little too big to fit.
Try as I might, I can't find the previous thread where there was a picture of one of these. Someone mentioned having found them in a facility where 120VDC equipment was manufactured. They were used for testing DC equipment, I guess.
I'd say you would have used these anywhere the voltage or frequency differed, or where for some other reason you didn't want people plugging in standard stuff.
I gather the NEMA configurations were developed in the '60s; people on ECN have said as much. Nowadays, there is a configuration for every AC voltage and class of service, but you might still have a special purpose. For example, in off-grid solar-powered homes, it is common to run low-power DC loads (cordless phone, answering machine, alarm) directly off the battery, to eliminate inverter losses. The usual practice is to use a 6-20R for 12VDC loads. Many AHJs will accept this if there are no 240V circuits in the house. 406.3(F) requires that receptacles connected to different voltages, frequency, AC or DC, be of non-interchangeable design, but does not specify anything further.
Here is a page from a 1930 book on US residential wiring, showing two different ways of grounding a washing machine. I'm guessing the perpendicular receptacle was the 20A version rather than the one in this thread.
Many AHJs will accept this if there are no 240V circuits in the house. 406.3(F) requires that receptacles connected to different voltages, frequency, AC or DC, be of non-interchangeable design, but does not specify anything further.
We have a similar situation in Britain, where it's not uncommon to find the old BS546 round-pin 2- and 5-amp (250V) plugs being used for 12 or 24V D.C. power.
I was sure that our older "code" editions contained a similar clause about not using the same type of connectors for different voltages or systems within the same installation, but I can't seem to find it at the moment. Maybe I'm getting confused with the American NEC.