... those old double tee receptacles were never intended for 240 volts or 20 amp equipment. The reason for the t slots existence was that in the early 1900s there were two competing 120v plug designs- the modern parallel and a 2 prong tandem Flat design and the t slot was simply a device that would accommodate both plug types.
The reason for the dual 125v/15 amp 250v/10 amp rating on old receptacles (many old recs that only accept parallels are marked the same way) is b/c lighting ckts were limited to 10 amps for ckts over 125 volts until the 1950s or so due to some rule in the NEC from what I've read.
There is no evidence that the t slot was ever recognized as a true Nema pattern and Nema standards changed in the 50s or 60s to where flat blade plug designs were reserved for 220 v or higher voltages to kind of give you at a minimum an approximate age of 120v equipment that has that old plug......
Beyond that, the old double tees were effectively banned by the 1971 NEC rules on interchangeability.
Yes they do still manufacture that old t slot and it's called a Leviton 5000-i and it, much like the beast in your link, isn't listed. The 5000-i is for replacement use only, but no way should they be put in- rewire, gfci options or a brand new 1-15 only where one is removed.
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