for sure a NSU "OSL", I am not sure about the exact model, probably a 500cc model..?
Link: http://www.hubler.de/nsu_501_osl/geschi ... ht_osl.htm
Btw, one of our motorcycle friends used an old NSU "OSL" in the early 1970s as a "Speedway" machine,
with very little success, but our friend could not afford a better bike back then...

Oha, the old times...

Hans1906
* I wish, to have just one of the OSL motors for a display here in the house, just the engine...
Video: https://www.youtube.com/watch?v=4AbbzrzQeCw