Hello Pat and Rich,
I agree - a most unusual machine, and almost certainly an NSU.
Note that it's a slightly different model than the one shown on 3wheelers.com. While that one has the 2-speed gear and clutch in the back hub (perhaps a ROC hub?), the one here has the NSU 2-speed gear and clutch on the engine pulley. The gear is operated by the vertical rod passing through the "N" on the tank to the little tram handle on the tank top. Wind it one way for low gear, then the other for neutral and then top.
Regardless of the exact specs of the machine, it's a most atmospheric photo! Thanks for posting it.
Leon