Tachometer, US Navy Mark V, Kollsman 621KN-04, 88-I-2500, SBD, TBM, F6F, PBY
Here is a very nice example of the US Navy Mark V WWII-/post WWII-era electric tachometer, made by Kollsman (part number 621KN-04), as used in aircraft of the US Navy such as the SBD Dauntless Dive Bomber, the TBM Avenger Torpedo Bomber, the F6F Hellcat Fighter, and the PBY5A Catalina Seaplane (which used 2). (See photos of the SBD and TBM instrument panels from their WWII-era flight manuals showing the instrument location).
It is in fine condition given its age. The data label indicates that it was made for Bureau of Aeronautics US Navy ("Bu Aero USNavy") and it has the stamped anchor symbol of the USN on its rear. The dial markings are clear and the glass and case are intact. Measures 3.23 inches diameter and ~5 inches deep.