Well, I´d go for the ca. 1929 model "4 RL", because this was the first year with a rectangular cooler mask and hood.
It has been built in significant numbers for the german Post and has been used as a basis by several coachbuilders like August Nowack AG in Bautzen.
Maybe this is a 4 RL chassis on the way to a coachbuilder?
You seem to known the driver´s name?
Then I´d go for Dr. Alfred Haesner, one of the constructors...