This is a German Drehzahlmesser or tachometer overhauled during the war by the German Air Service's Flugzeugmeisterei. Tachometers made by Wilhelm Morell and other manufacturers were overhauled after a number of flying hours and when this happened the original manufacturer’s dial was replaced with the Flugzeugmeisterei’s own dial with their serial number, in this case No. 8146. As Koloman Meyerhofer kindly explained to me, "The scale is 0-16 which indicates rounds per minute for a maximum 0-1600. German engines ran normaly at 1400 to 1450 rpm."  Seen in close-up below, the glass is etched with a semi-transparent "P.u.W." with a winged propellor and the number 12677.  P.u.W. stood for Prüfanstalt und Werft, the testing station and workshop section of the Flugzeugmeisterei.   Also below is an enlargement of the approx. 4 mm long lead seal crimped onto the instrument to indicate that it has been inspected.  The photo of the side of the tachometer has a small arrow indicating the location of the lead seal. The metal ring with 4 holes allows the instrument to be mounted to a wood instrument panel.