If I were Lexmark, I'd leave out the wires that aren't used, but I would include the unused pins just to give a bit more keying to the connector. Unless there were some workstations that had some of the holes missing in the PS/2 keyboard port, to stop people plugging in the mouse by mistake? Such a workstation would need a missing-pins connector on its keyboard -- but would then become incompatible with standard PS/2 keyboards.
I do dimly recall seeing something like this on Greenock-made Model Ms but I could be just imagining that.
(IBM's biggest design mistake in the PS/2 line, in my view, was to make the keyboard and mouse plugs identical but not interchangeable.)