Tachometer, Electrical, 400-3600 RPM, Luftwaffe Fl.20269 Drehzahlanzeiger
Here is a nice specimen of a WWII-era electrical Tachometer, or Drehzahlanzeiger, as used in aircraft of the German Luftwaffe. This tachometer received electrical signal from a generator located proximal to the engine drive to indicate engine speed, from 400-3600 RPM. Likely made by Horn. The Fl number is illegible, but the instrument is identical to that listed at the reference site GermanLuftwaffe.com.
It is in reasonable condition given its age. The glass and face are intact. Measures ~3.25 inches diameter and ~3 inches deep.