they are this.   
the front axle has a turntable on it's top, so it's like bolster steering, the whole differential rotates.
so the premise of the concept was that in combination mode (front steering,+articulation)   you could turn the steering wheel about a half turn each way from center before the tractor would articulate.( about 7 degrees of steering each way).     so you can run row crop implements with these machines.  also can run them in articulation only mode.          91/92/93 series steiger tractors in the 10/30/50 models can have the front steer capabilities also.