Hello Peter,
I found something about your uncle in a book about German car engineers:
1874: Born in Berlin on September, 18
At the end of the 1890s: employed at Sté. Gobron-Brillié at Boulogne-sur-Marne, France
Around 1900: employed at Nagant Frères, Liège, Belgium
1900 - 1905: Chief engineer of the Berliner Motorwagenfabrik GmbH (BMF), Berlin-Tempelhof
1905 -1908: Chief engineer of the automotive department of the Siemens & Schuckert Werke, Berlin
1908 - 1910: Chief engineer of the Motorenfabrik Protos GmbH, Berlin, which was taken over by Siemens-Schuckert in 1908.
1910 - 1911: Valentin worked in his own technical bureau (Technisches Büro für Fragen der Automobil- und Flugtechnik)
1911 - 1912: He worked for Deutsche Automobilindustrie Hering & Richard, Ronneburg, which built the Rex-Simplex cars
1912 - 1914(?): Head of automotive department at Russko-Wagonyij-Zawod, Riga, Russia, which built the Russo-Balt cars
1919: He was to become the first "Reichskommissar für das Kraftfahrwesen" (National commissioner for motor transportation)
1921: He founded his own publishing house
1950: Death
Rex-Simplex cars were built from 1901 to 1914, trucks until 1921. A Rex-Simplex 10/28 PS from 1913 has survived and is displayed at the Norsk Kjøretøyhistorisk Museum at Lillehammer, Norway.
Unfortunately, I couldn't identify the pictured car, but if the picture is from 1908, it probably is a Protos. But that is just a guess.