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 I would like to think they have been replaced by now because it requires a whole lot of care by the user that they know what they can plug in.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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On Fri, 06 Oct 2017 02:17:55 -0400, snipped-for-privacy@aol.com wrote:

Hospital grade outlets and cord end receptacles have that configuration, and I always figured it was yet one more insurance that a device has proper phasing and fault protection when it gets plugged in.
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On Thu, 18 Jan 2018 09:11:45 -0500, DecadentLinuxUserNumeroUno

You are talking about a standard NEMA 5-20 (120v 20a) where one side is a T slot. The NEMA 6-20 (240v 20a) is similar but the T is on the other side. The old one we are talking about had a T slot on both sides.
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snipped-for-privacy@aol.com writes:

Yes, my outlet looks like it would accept any of NEMA 5-20, NEMA 6-20 or NEMA 6-15 plugs if the ground pin was removed.
I was wondering if it was intended for either a 110V or 220V circuit (and the homeowner had to remember which plugs could go in which outlets), maybe for DC, or perhaps before they standardized on what's now the NEMA 1-15 plug.
NEMA 2-15 (220V, 15A, no ground) is in the spec, but was it ever used?
I also have duplex outlets with the same T configuration, from a house built in either 1940 or 1950.
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On Wed, 31 Jan 2018 18:52:39 +0000 (UTC), snipped-for-privacy@world.std.spaamtrap.com (Michael Moroney) wrote:

I suspect this was before NEMA had really established the standard and the manufacturers made a receptacle that would take whatever you had. I don't think there was really that much cord and plug equipment running 220/240 15/20a anyway in those days. It really did not become a thing until we came out with the window shakers and by then grounded plugs were pretty much standard. I know in 1956-7 when we got our first Fedders I had to run a new circuit with a NEMA 6-15 and the smaller one we got for the bedroom needed a 5-15. The house used 1-15s but fortunately there was a ground present in the box.
You still had ranges and later dryers back in the olden days but they used an entirely different plug. The crow foot range plug (10-50) has been around since the 20s. The "L" ground dryer plug (10-30) was just an adaptation of that.
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Michael Moroney wrote:

It is 120 - 240 It has not been 110 volts for SEVERAL decades.
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I know that. I deliberately wrote 110-220 because at the time they defined NEMA 2-15 it may have still been 110v, or 117v. Nameplate voltage has risen over the years (110, 115, 117, 120).
When the outlet I asked about was defined, it was almost certainly still 110v.
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Michael Moroney wrote:

Nope... That would have been pre WWI.
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