Thank you Dick, Mickey_Fx and Kay_supply for your answers about this "mystery shift". I can add an information about this square button which is on both rear mudguards : There is two Japanese ideograms near it (have a look on the picture) : an ideogram of one sort corresponding to the situation where the button is pushed and an ideogram corresponding to the situation when the button is pulled. I have found the same ideograms near the command 2WD-4WD of my tractor so they mean 2WD and 4 WD.
The question is : why do I have to push (or pull) on these buttons when I am in 4WD or 2WD ? It sound like if they have some action on the rear wheels....
Any idea about this ?
Phil