Take a peek through the speaker grille in the back, see if the backplate is a shiny gold/brass color.
To use that cable, you'd need an AT->PS/2 passthrough adapter, then a PS/2->USB active adapter like a bluecube (unless your PC has native PS/2).
You could also do your own custom converter ala Soarer, but that's a bit of work. Not terribly complicated, just... work.
Or you can just get a converter cable that plugs right in:
http://www.ebay.com/itm/USB-to-SDL-6-Pin-6ft-IBM-Lexmark-Unicomp-Model-M-Clicky-Keyboard-Cable-NEW-/301654850280 which will allow you to reprogram the keyboard as well.
That's from orihalcon who is a member here, would guess he likely sells them slightly cheaper directly so you may want to contact him here.
You can get replacement caps from Unicomp.com (including colors) or on ebay. Some here may also set you up on the cheap. In the meantime I'd move the numpad 2 to the down arrow position.
That is a nice example of a classic M.
