T-shaped US outlet?

I have an old (1940s I think) ceramic outlet in a box, for a two prong plug, no ground. It accepts a standard US 2 prong (1-15) plug. It is
polarized. It is also marked "15A 125V" (molded in the ceramic).
The box also has 2 standard fuse holders, one in series with each slot.
However, both slots are T-shaped, like one slot is in a 5-20 or 6-20 outlets. Almost as if it was designed to accept a 2-15 plug (does such a thing exist?) as well as a standard 1-15 plug.
Why are the slots T-shaped? Were 2-15 plugs a thing for 220V at the time, they installed the same outlets on both 110V and 220V circuits, and you just had to know which outlet to plug in the 110V things and which to plug the 220V things in? And if you got it wrong, too bad?
That's what I thought when I saw these in older houses. But this one is explicitly marked for 125V max, so it's not intended for 220V.
For a 110V circuit, one fuse would be in series with the neutral, not a good idea. But it would make sense if used on a 220V split phase circuit.
Why are the slots T-shaped?
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On Fri, 6 Oct 2017 02:12:15 -0400, "Anon Y. Mouse"

There were receptacles many years ago that would accept a 1-15 or a 2-15. I would like to think they have been replaced by now because it requires a whole lot of care by the user that they know what they can plug in.
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On 10/6/2017 12:17 AM, snipped-for-privacy@aol.com wrote:

As everyone probably figured out, this is US.
Used to call them T-slots, and they were used to replace 120V receptacles where there was no ground. I have several in my collection, by Leviton, P&S, Homart. There is a picture at the font-of-all-knowledge, second pic down, right end. https://en.wikipedia.org/wiki/NEMA_connector
One of mine (and only one) is labeled 15A 120, 10A 250V, and is UL labeled. Marked with patent number "1591773-1591707". I assume that isn't useful, I am too lazy to look.
I have never seen a 2-15 plug for 240V. But long ago I found a plug like a 2-15 intended for 120V, so I believe T-slots were for compatibility with ancient plugs. Likely weren't 240V receptacles back then.
I forgot the parallel slots were polarized, all of mine are. The Ts aren't polarized.
I once had an ancient vacuum cleaner with a plug like an Edison lamp base. It would turn freely with respect to the cord until it was tight. I assume in the real early days you could remove a light bulb and screw in the plug.
Probably matching the above, somewhere in my collection is a 'receptacle' that matches the Edison lamp base plug. Mounts the same as modern receptacles. Nice brass plate with a brass door that opens.
Would think "standard" fuses would make the device rather large.
First place I lived away from home was a duplex with a 30A 120V service. The service and 2 branches had neutral (plug) fuses. There used to be a solid brass shell with ceramic center with a slot for a screwdriver that could be used to eliminate neutral fuses.
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bud-- wrote:

Very early some places had DC.
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