They changed the body style to add a large stowage box
" Beginning around 1935, most vehicles were equipped with a Kfz. 15 car body with a large stowing box at the rear."
To actually be a Kfz. 15 it needs some radio/telephone gear added.
I get the impression that cars for other usage than Kfz.15 got the new body style, everyone wants more stowage space …
http://www.kfzderwehrmacht.de/Homepage_english/Motor_Vehicles/Germany/Auto-Union/Horch/Horch_830_R_Kubelwagen/horch_830_r_kubelwagen.html