Maybe one of these?? http://www.mirab.com.pl/dkw500/mirabengdkw.htm But this page http://www.ozebook.com/dkw.htm shows it as a 350. A mystery but a rare bike anyway. DKW RT 175 '36- SB 500 Normal http://dkw-motorrad-club.de/bikes/v45/bilder/SB-500-9-1937.jpg
Motorcycle license plate B9129 or E9129(?). B = Stockholms län and E = Östergötlands län. Car license B9129 was on a 1939 Ford car in 1957. Car license E9129 didn't belong to any car, truck or bus in 1959.