The On-The-Stick was long since discontinued. All Unicomp buckling spring trackpoint keyboards use that force sensing resistor pointing stick which has the wobble in it.
The electronics for the Unicomp pointing stick aren't going to be compatible with the one in newer IBM keyboards. And the Endurapro has an arrangement whereby the keyboard controller plugs into the pointing stick controller, which then has a USB or PS/2 cable coming out of it. The PS/2 Endurapro has a PS/2 passthrough though, so you could tuck the controller from the IBM keyboard inside it and hook it up to the passthrough port.
