Hi Pierre,
I had the chance to open a friend's defective M3Z in standard configuration.
I found out the (4) phillips headed screws from the top side hold the 2 visible lenses to the housing.
The slotted (5) screws hold the 2 guiding rods of the zoom sliders.
So there is no chance to open the microscope housing by removing them (that's what you wrote in the beginning).
Also found out the microscope body is not fixed to the stand using the 2 screws from the objective side (locations marked with 2 blue circles on the photo a few posts before). I could slide a thin paper in the small gap between microscope body and stand at the locations the 2 screws should be. But the paper could slide in without resistance, so no screws at that locations.
Tried to get help from Wild Heerbrugg service, but nobody knows that model variant nor how to remove the plate...