I was able to source a replacement for my PC Engine adapter - this wasn't so hard to source (but center-negative wasn't easy to find either)... it seems that Casio keyboards are using this same type.
I have seen Yamaha keyboard adapters with the same "reverse polarity" (tip/center-negative) and tip size. 5.5x2.1mm is considered the standard tip size for most everything from routers to CD players to, well, everything, but the world has pretty much settled on center-positive tips as the standard polarity. It's slightly safer to prevent shorting voltage to ground, since ground is often accessible all over the exterior of a device.
The reason Japan moved away from the standard 5.5x2.1mm size is because it was already normal to sell devices without AC adapters there several decades ago and they needed to prevent users from plugging those into newer devices after they transitioned to center-positive (in-line with the rest of the world). This meant chosing a physically-incompatible connector.
The story goes a lot deeper when you look into why they sold consumer electronics without adapters and why they settled on center-negative in the first place.
After World War 2, Japan was not the consumer electronics power-house that they would become by the '80s, but a quirk in their power infrastructure led to them engineering consumer electronics a bit differently than the rest of the world. They needed most domestic electronics to run agnostic of line frequency because some regions had 50hz power and others had 60hz. That meant engineering more devices for DC that would have otherwise take AC.
Since these devices were already DC, they had incentive to make devices portable and battery-operated to save the cost of including an AC adapter. It also meant they could sell many of their devices around the world with little to no internal changes, giving them an advantage over foreign CE manufacturers who often focused on their engineering on domestic power requirements first. If the device was battery-operated then Japanese MFGs didn't even have to include a different adapter plug to sell it in other markets! This is why Japan became known world-wide for compact, portable, electronics. This did not mean you could not plug your portable stereo into the wall... it just meant that it often did not include a wall plug... especially in Japan. They just didn't have that expectation.
Though Japanese manufactures still tried to scare customers into only using their official adapters with their products, there was a ton of cross-compatibility, and consumers expected it. 9v center-negative with a 5.5x2.1mm plug was easily the most common. In order to save costs, lower prices, and reduce waste, it even became acceptable in Japan for a device to not include an adapter when it was not portable and required an adapter. That's because you may already have a compatible one and it was seen as wasteful to have one for every device. It reduces cost, shipping weight, etc. Consumers balked at a product that required a less-common plug that wasn't included, especially because the manufacturers justified leaving it out with promises of lower prices and less waste. Thus, 5.5x2.1mm center-negative became a pseudo-standard and a consumer-expectation unless there was a good reason.
This is why the Super Famicom was designed to accept the Family Computer AC adapter. Nintendo's logic for not including one while warning customers not to use another brand was that there was a good chance you already had one from your Family Computer and there was no sense increasing the price for everyone for the people who did not have a FC power supply handy. This is also why the Virtual Boy Adapter Tap in Japan fits 5.5x2.1 center-negative Family Computer adapter but takes a SNES plug in the USA (SNES is center-positive and doesn't physically fit). This is also why they sold the tap without the adapter. They did sell an adapter tap "Set" here in the USA and maybe in Japan too but the tap-only was also a thing. Internationally Nintendo did not leave the adapter out of a retail device that required one until the DSi XL in EU/AU and the New 3DS XL in the USA. I was one of the people who called and complained, since there was no reason to keep a DSi, DSi XL, 3DS, 3DS XL, 2DS, etc around when the 3DS played all of their games and they were forcing someone to buy an adapter regardless when you trade the old one in or pass it on to someone else. "Blatant cash-grab." Literally the only other thing that used it was the mail-order-only official Wii Remote charging stand (never sold in stores) which was already ridiculously overpriced, didn't include the expensive batteries, and didn't include the adapter either IIRC.
It seems I wasn't the only one to complain in the USA since we are the only region that got USB power bricks with our Classic Edition mini consoles.
On that note: Notice how the Japanese Family Computer Mini and Super Famicom Mini use the same official sold-separately USB brick in packaging styled after the Famicom Family HVC-002 adapter?
History truly does repeat itself. The SHVC-CPU-01 SNES and SFC motherboards only differ in the RF modulator and DC jack, and the DC jack is purely because they were doing the same thing in Japan way back then.
But how does all that explain their affinity for reverse polarity despite concerns with having the positive side exposed? Well, Dave Jones (EEVBlog) explained that for battery-operated devices that also have an optional three-terminal DC input jack you, the jack could also be used as a physical switch for disconnecting the internal batteries when using external power. It didn't require any special circuitry as long as you used center-negative. With Japan's affinity for portable DC electronics with both options (battery and wall adapter), you can see why this carried more influence there. Center-negative became the standard in Japan while the rest of the world settled on center-positive.
Obviously, we don't need a dumb switch these days as much as we did decades ago and conformity is a bit more valuable than it once was. In the early '90s the Electronic Industries Association of Japan and Japanese consumer electronics manufacturers saw this coming and decided on a standard for most plugs that would be center-positive going forward: EIAJ.
You will notice that most Japanese game consoles were revised to be center-positive in the early '90s (typically a yellow tip). Japanese CE companies did not want their customers who were accustomed to reusing plugs to try their old center-negative PSUs in their center-positive consoles, so they settled on EIAJ-03 for most with a 4.75mm barrel and a 1.7mm center pin. Since the rest of the world had already been using center-positive 5.5x2.1mm for decades and did not have consumers conditioned to share random plugs, they mostly continued using that size, but there was already an equivalent "Philmore TC275" tip size that we also used in similar situations where you didn't want users trying the wrong plug.
The outer diameter of TC275/EIAJ-03 is smaller, which prevents you from inserting an old 5.5mm plug into a newer console. The center pin is smaller (1.7mm) so that you can't physically force the new, smaller, plug onto the larger pin of an older console. It is also plastic-tipped so that you don't short the center pin (ground) to the center of the plug (positive voltage) while attempting it anyway.
Examples of 5.5x2.1mm center-negative consoles from Japan:
Mark III/Master System
Mega Drive/Genesis 1
Mega CD/Sega CD (1 and 2)
Game Gear and accessories (Domestic)
Family Computer (NES-compatible)
Super Famicom
Virtual Boy Adapter Tap
PC Engine/TurboGrafx
CoreGrafx (and CoreGrafx II, PC Engine Shuttle)
CD-ROM² and Turbo-CD drives (stand-alone)
NEO-GEO AES (regulated 5v and unregulated 10v/11v!)
Other center-negative Japanese consoles before the industry shift:
SuperGrafx
CD-ROM² Interface Unit/Turbo-CD dock
Examples of 4.75x1.7mm consoles from Japan after the change to center-positive:
Genesis 2
Nomad
Game Gear (International)
Multi Mega/Sega CDX
PC Engine Duo-R, and Duo-RX
PSone
PSone LCD
PS2 slim
Other center-positive Japanese consoles after the industry shift:
PC Engine Duo / Turbo Duo
Super CD-ROM²
Mega Drive 3 / Genesis 3
Except for consoles designed to use their predecessor's plug (Core Grafx II, Virtual Boy Adapter Tap), you can see that a clear shift happened in the early '90s... then we started to get consoles with modular or built-in AC power supplies (Saturn, PS, N64, DC, PS2, XBOX, PS3, etc). Even those would often get EIAJ-03 in their more portable "slim" revisions (PSone, slim PS2, etc).
Of course, you also have the systems with non-standard sizes specifically to keep you from using the wrong adapter. Turbografx Turbo-CD and the PC Engine CD-ROM² Interface Unit, for example, were all still center-negative. I carried my first Turbo-CD dock into Goodwill Thrift Store with me and tried every plug until I found one that fit. I found a 12v Hyundai adapter that looked like it was for an LCD monitor or something, so I reversed the polarity and it worked.
Actually, it works a bit better than my original Turbo-CD power adapter I got later, since some combinations of my collection of Interface Units, Turbo-CD docks, CD-ROM² drives, and Turbo-CD drives do not work when I use the original Turbo-CD adapter and DO work when I use the slightly higher-voltage Hyundai plug.
I brought up that example because the Turbo-CD dock and CD-ROM² Interface Units were designed to power the original consoles but needed additional power. Their higher-amperage plugs were deliberately made so that they would not fit the original 5.5x2.1mm plug, even though they were still center-negative. Your original plug wasn't wasted though: both the CD-ROM² drive and the Turbo-CD drives use the original console adapter when used as a stand-alone CD player... yeah, maybe that's not as compelling a reason to keep the original adapter around today as it was then.
I think it's interesting to note that even though Sony was in a similar situation with the PSone + LCD requiring more power than the original PSone adapter could comfortably supply, they made no attempt to change the adapter sizing. The PSone came with a 2A, 7.5v SCPH-113 and the LCD included a 3A, 7.5v SCPH-121 with identical EIAJ-03 plugs. The adapters look identical except for that tiny spec and model number difference and many users saw no reason to even take the new adapter out of the box... especially since the old one appears to work. Maybe it's fine and Sony just didn't have enough headroom for some regulatory requirement so they threw that in for compliance.
The SuperGrafx sold so poorly I guess NEC wasn't worried about people using their center-negative adapter on the center-positive Super CD-ROM² and PC Engine Duo, which are all the same 6.3x3.0mm size. Maybe they had a huge stock of those parts from when they were still expecting the SGX to sell, so they re-purposed them in the original Duo.
Universal adaptors with a 2-pin removable tip design are all polarity reversing, and I see quite a lot of those (and own a few). The downside is that they're not as secure, but you could always glue it in if it's going to be exclusive.
*edit* well that's weird. Some of those adaptors I looked at say the polarity is fixed, but I was sure that wasn't the case.; Maybe it's a mistake in the listing? This one doesn't explicitly say it's a switching PSU, but according to one review it does reverse polarity.
Yeah, that's because they made one pin bigger than the other so users who aren't paying attention would not get it backwards for most devices (designed for center-positive, of course). Even the ones that aren't keyed are often marked with the assumption that you are using center-positive, like the adapter cable Console5 sells with a 5.5x2.1mm plug on one end and a 2-pin Adaptaplug connector on the other. This is sold primarily for using with a Genesis 1 PSU but, IIRC, the marking for positive is only correct if you are using it with a center-positive adapter.