Hi Gerd:
Unless you know the answer already, it would be interesting to pursue this one to the end.
I am thinking that the truck was intended to tow something "special" or in "special" events.
I have in books etc. and in my docs files many photos of Faun zugmaschines over the years and none have this sedan/lux sort of cab.
Based on the lettering on the side of the cabin, which is illegible to my eyes, it looks like the first word might be "Gebruder" but I cannot make out the second word or the rest of the writing. Also of note is the placement of two flags with some sort of crest on the width markers and that same crest is repeated under the cabin writing. Maybe a Circus vehicle, maybe a special vehicle for parades, I just don't know but for some reason I would like to find out. It also looks perhaps like a Faun advertising piece which may help.
Let me know if you have the answer or maybe some tips to go further.
Bill