So, I bought a working 1989 Northgate OmniKey Ultra with complicated white alps, and it came with a cable that I, mistakingly, took to be a ps2 at one end when I first glanced at it. Then when I brought it home, it didn't connect to my motherboard ps2 port. I really want to use this keyboard, so maybe you could help me identify what kind of cable this so I know what adapter I need to get.
Here's the cable:

This is the side that connects to the keyboard. (I know this is ADB):

And this is the side I am having problems with. It's supposed to go into the computer:

It sort of looks like a ps/2 cable (aka, mini din 6), but it's "missing" a pin. It has five pins. Also, the middle rectangular piece is drop-dead-center, unlike a ps/2. What the heck?! ;D
Appreciate the help.