An Illustrated History of Military Motorcycles: 100 Years of Wartime Motorcycles, from the First Machines of World War I to the Diesel-Powered Types and Quad Bikes of Today, with 230 Photographs: Amazon.co.uk:
NSU - OSL 251 - 250 cc - 1950 - Catawiki
Incredible Restoration for 1937 NSU OSL 251 Bike - autoevolution