I can describe how it works on a PAL (later model). The centre hole is threaded and is what ultimately retains the boss. The boss itself has an oilite bush in it. A top hat washer whose minor diameter is the same as the projecting quill shaft is used. A spring is used between the boss and the brim of the top hat.
The boss has one projecting pin on it which provides the drive. This, in combination with the spring, allows the handle to be repositioned in 90 degree increments.
In principle, it operates in the same way as the knee crank handle but with fewer (only four) positions.
See:
http://www.vintagemachinery.org/mfgindex/imagedetail.aspx?id=17685
On my own, I made a new boss and now use a Moore jig borer-style steering wheel (originally from a boat).