Sinn NaBo 21 ZM aviation clock dated May 1975.
Good visual order but not tested if working.
Helmut Sinn Instrument clocks: The aircraft clocks of Helmut Sinn were at first very similar to the Junghans clocks of WWII., they were developed from them.
NaBo 21 ZM dashboard chronograph with central minute hand and two big pushers for start/stop and push back. The movement is based on Valjoux cal. 5. Case and shape like BoUk 1, winding and adjusting via knob. The clock has a central chronograph hand, a central 60-min.-counter and addition counting feature.