Analog vibrating reed frequency meter. That's an analog frequency meter based on multiple metal strips (reeds). Each of the metallic strips has a slightly different mechanical resonant frequency. It also contains coils to be connected to an AC current. 
<p>The metallic strips, with a resonant frequency very close to the frequency of the current in the coil, will vibrate in order to indicate the frequency on the scale. This device has been used to measure the frequency in the range of a 25Hz up to 105Hz, on 3 independent scales: 25 to 45Hz, 45 to 65Hz, and 65 to 105Hz. The input can be selected to accommodate a large of measurement AC voltages: 50 – 250Vac.
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Made by Metra Blansko, Czechoslovakia, 1950s.
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