It took a long time to identify the right apartment, then recently Sébastien Haarig provided me a series of new photos and, among them, the blueprints of the building at Joachim-Gottschalk-Weg 10. They were just there, hanging on a wall next to the elevators.
Two apartments on five have “two room and half” as mentioned in the book. The one on top-right and the one on top-left. But only in the one on the left, the smaller room has a window. In the apartment on the right, the smaller room gets to the balcony.
The smaller window in the smaller room is the right one.