I'm covetous of that board. Those dust covers should have been a required feature of all alps keyboards.
Did anyone, ever, make standard 5-pin din terminal keyboards to begin with? If they had, what are the odds that they used a proprietary protocol yet left the AT/XT switch intact? I'm thinking he may just be mistaken.
Personally, I just discovered that my recently acquired white/beige label Zenith Z-150 works perfectly, in Linux or Windows, on an HP 8300 SFF desktop and my old Windows XP Compaq. On every other system I tried, it had varying degrees of function, regardless of operating system. This is in spite of the fact that it doesn't work at all with Soarer's or Hasu's firmwares. I would take the board a part and check all of the traces and components for signs of damage, or shorts. If there's nothing, maybe you just need to try a few more adapters until you get lucky, or build/buy a proper Soarer's converter as it apparently works with these boards. Do you not own any desktop computers with ps/2 ports to try?