Hi guys! sorry for necroposting but i have new theory about the "red button"
Anyway i was reading this.. http://www.vintagewings.ca/page?a=1261&lang=en-CA "I noticed that this particular aeroplane incorporated a small electrical switch on the floor, marked “Prop: Auto/Manual”, but it was wired to the Manual position"
So.. i think that the red button could be the switch for selecting Automatic/manual prop, i know that he says that the button is on the floor but i have read that the button was placed on the throttle or on the dashboard originally.
Maybe i am just barking up the wrong tree here
you can just tell me if so
Osram i looked up nullschub, it also means "zero-thrust", maybe it could be something with the prop?