Great question — answering as the UphillTech team:
I too was curious about that, and did a small amount of research. I found this forum post which makes me think that you just "had to know" what voltage the receptacle was wired for. I suspect you would know that based on whether you lived in a 110V area or a 220V area (These receptacles were first used back in the days of 110V/220V era.) We live in an era now where we have an entire continent on standardized voltages and frequency, but that was not always the case. In the early days of electric power, there were numerous independent power systems that operated at different voltages and frequencies. More background here
Full answer by Ken Carlson: https://diy.stackexchange.com/a/331991 (CC BY-SA 4.0).
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