These configurations are all standard and well defined. I believe GE has a
better, more comprehensive chart but 15 seconds of Googling got me this one.
Each current, voltage and phasing capacity has it's own defined receptical
I thought they may have been some electrical people here.
The National Electrical Code is the basic code for the US. It is adopted
(sometimes with modifications) for most of the country.
The NEC requires, in general, "equipment required or permitted by this Code
shall be acceptable only if approved" (110 2).
Approved is defined as "Acceptable to the authority having jurisdiction."
That is the inspector, or whatever adopted the code. UL listing will work.
Listing by some other major testing labs probably will also. Testing here
means samples of the equipment have been tested by the lab to determine if
they meet the applicable standard. For switches, receptacles, ... the tests
include many operations of the device, so it not only fails safely, it has a
useful and safe life. Europe, I believe, the equivalent 'listing' process
usually means the manufacturer certifies the device meets the appropriate
Further, for receptacles, the NEC says the "receptacles shall be listed...."
=================We used to call the receptacle you found "t-slots", and they were the
standard receptacle used for replacement of 120V ungrounded receptacles. I
looked in my junk, er.. salvage, boxes and found 4 of them. At least one is
I think I remember from the dim past one plug on something that had 2
horizontal blades. I suspect that the horizontal parts are for compatibility
with real old plugs.