Be careful here... Most of these IEC plugs like this in Asia or Europe use 250V, 50Hz supply.
So, if your machine transformer is set up for 250V, 50Hz and you energize at 120V, 60Hz, not sure what the results might be.
Check the nameplate or ratings on things before you turn that baby on.
Just my 2 cents...
You can purchase a universal travel converter that might carry your machine with the normal 120V outlet voltage.
There are many varieties and brands of these available at different prices and shapes, but here is one:
https://www.amazon.com/DOACE-C11-Straightener-International-Transformer/dp/B07DQLM1P1/ref=sr_1_5
Edit here: this one will not work because it is designed to plug into 250V outlet and energize 120V devices.
Your machine could be a 250V device, so this converter is not appropriate, I don't think.. But you get the idea...

It appears, based on the schematic in the link provided above that you can relocate the red-green wire to the 125 Volt tap on the transformer and run the machine on 120V. Note that there is still a courtesy plug shown on that schematic that would still be energized at 225 Volts, unless the tap for that plug is also moved on the transformer on a yellow-blue wire. I would advise extreme caution in all of this and fuse everything prior to energizing. Especially, if the previous owner has already made some adjustments here, that just makes things more interesting. Just be careful and don't have a shocking experience. I hate it when the smoke is released.

Perhaps it used to be red/green and you can't see the green stripe anymore.
Looks to me like you are right... Someone already adjusted this wiring...
I see 3 blue wires under that lug... Perhaps one of those goes to the convenience outlet.
I would check that wiring and check voltage at your convenience outlet also before plugging anything in there.
Also, be aware that other taps on this transformer not being used are still energized up to the 230V tap.
So, some of those exposed wires could be the energized taps on the transformer that are no longer used and they appear ready to be touched.
I would tape those or cover with wire nuts to avoid any issues where someone accidently touches that or drops something on those.
This kind of thing scares me, cause I don't like getting shocked without it being on purpose.

On 2nd thought, that red/green wire is likely the 230 Volt tap on the transformer.
This whole thing scares me, cause it just looks nasty...
I cant really see all the wiring or all of the schematic, so I still advise major caution.

Upon further review of that schematic, there is a red wire shown on the low side of the transformer to a mid point on a 32 Volt AC circuit for something that has several connections shown. It appears to be on the 30 Volt tap. So perhaps that red wire is not the original red-green wire to the 230 Volt tap.

One other thing I noticed is that this 230 Volt circuit is depicted with a 6 amp fuse in it on the schematic provided.
So if this machine is now operated at 120 Volt, it is possible that fuse might need to be little larger.
Maybe like 230/120*6 which is 11.5 Amps... Maybe a 10 Amp fuse would be sufficient.
Perhaps that has also already been replaced. But, I would suggest that you check that...

Nice work... That sure looks a lot better... Glad nothing exploded and no one got hurt. Now you can work on small things that don't normally cause major problems. You know... lights, coils, plastic, rubber, all that good stuff. Good luck with all those Jones plugs on that machine. Those can really be a pain sometimes...

I really hope that you can make this game, "Last Lap", work... I watched some of the video archive gameplay on this machine and it is very unique. Some of the advertisements are in espanole'. Good luck getting original parts for that... The sound was tones like beeps and boops. Seemed to play fairly fast too. Should be fun if you can make all that work. I don't think I have seen another similar game, but I also never been to Spain and played pinball there.

interesting... Schuko... Hadn't heard that term for those recessed plugs... In the USA we are governed by the NEC (National Electrical Code). Most devices require a 3 prong plug, with a ground lead on the 3rd prong. If a ground is not required, a 2 prong plug is acceptable. We have a few other plugs for large load devices, like a stove or dryer, but they are normally used only for larger loads, like a trailer, or mobile home. A lot of Americans might never see Euro plugs, unless they are exposed to IEC code or travel internationally. So, a plug like this from Europe is unfamiliar to most Americans and it definitely will not fit a normal outlet here without an adapter. USA also has all 60Hz devices in all 50 states, like clocks, motors, etc, etc, so 50Hz is not on our radar either. Also, thanks for the color code info. That might come in handy one day...

I guess you will need to get that multimeter back out. I suppose that you have to expect some issues on a machine made during this time frame. I would be willing to bet that the problem has something having to do with those Jones plugs.