So, this is an NGS keyboard. I'd never heard of NGS, but they seem to have been around early enough to make some boards with Alps switches, and they're still making keyboards now (only domes, unfortunately). Going by the stray bits of Spanish on their website they seem to be Spanish-based!
And the board is Spanish-layout. Those stepped caps are a bit unusual, but not much else is special so far. But under the caps...

The 1.25u caps have longer pinions to match the cap. It looks as if the spacebar has several small pinions.
The caps have little teeth on them to mesh with the pinions. I suspect they must have been fiddly to put back on... though probably no worse than a scissor switch.
Essentially this is another means of stabilising the keys by restricting their ability to move off-centre, rather like scissor switches. But this is probably easier to scale up for a longer travel.
To quote from the owner's description: "It's very smooth to type yet it's difficult to accidentally type other keys. It's also very confortable to move your fingers between keys, and you don't need to rise your fingers to do it. About key travel: it's about ~3mm. Compared to my laptop: 1mm and my standard keyboard: 4-5mm." I can imagine a small amount of friction from the pinions would have the effect of smoothing out the bump from the rubber dome. Possibly there's a little binding when the keys are pressed near one end, which would make an accidental off-centre press less likely to produce a typo.
I asked if the pinions got dirt in them: "And yes, dirt gets stuck in the pinions, and consequently, keys get stuck too. But it takes many, many years and it's 'easy' to clean."
The owner thinks the board was bought in 2007 - and the .eu domain wasn't approved until 2005, so that gives us a limit on the date. I'd surmise that this is someone's attempt at improving on scissor switches (perhaps Elecom's more likely than NGS) - but how it got between Japan and Spain is another question. I think it could have rivalled scissor switches if it had come along at the same time, despite the dirt issues.
My heartiest thanks to viladrau on Reddit for providing the photos and information on this fascinating keyboard!
