Unicomp was never advertised as a dualie, AFAIR.
Ok, I found where I read about the G84-4100. And the mystery is solved, thanks to The Keyboard Company!
It's the xCxxx model (the combo PS/2+USB) that support this. They, like the other dualies, sport USB connector and include USB-to-PS/2 passive adapter.
The xPxxx models are PS/2 only. They need active converter.
Links:
xCxxxxPxxxThanks, Ripster. You were helpful in an unexpected way.