Filcos are really damned good keyboards.
I'm not a complete expert on this, but essentially one of the huge selling points of a high-end PS/2 keyboard is that using the PS/2 bus allows for what keyboard nerds call "N-key Rollover;" this means that you can never cause ghost strokes or keys to stop registering by hitting too many keys too fast—this is a limitation that USB keyboards will always have.
Anyway, I don't really know about how the specifics for the USB & PS/2 system work on high end keyboards like that. It has a mini-USB port in the back of they keyboard and you plug whichever cable you want into it, right? This is a complete shot in the dark, but I would guess that they are all true PS/2 keyboards with what is essentially a single PS/2 port replicator built right into them as a means of gaining full USB support.
If any of you are really interested in completely nerding the hell out about keyboards, I can recommend the fine folks over at the GeekHack forums. Things you will never have imagined wanting to know about keyboards will be made clear and fascinating there. This is also where I learned how to paint keyboards.