At last I have found a very good schema, a simple drawing from Schwimmwagen's gearbox. There are some explanations in German, English, French and Italian. The Schwimmwagen used basic mecanic project of VW Sedan, however it was a militar vehicle, rude, strong, stout and amphibious. The transmission was similar but with some improviments, mainly in gearbox. So it was possible ride around in 4x2, 4x4 (4WD) and using cross country gear. The gearbox was quite unsynchronized, perhaps a bit awkward to drive. 4WD and cross country gear were possible only with first speed. Perhaps this gearbox has seemed TOP SECRET but now is clear up.