This has to be the oldest bit of kit i have, this used to belong to my dad,
and has been up in the loft for many years (after his passing),
i think it's from the 1940's, used in the 2nd world war, for the U.S Army,
it would of been powered by a 135 volt battery (but at some point had been converted to run off AC),
from the info i have found, this was used for calibrating radio equipment,
by using a method called Zero-Beating, the Frequency Meter still works,
but the dial reading's are a bit off from the setting in the Calibration Book,
i did replace some of the components in the power supply to make it a bit safer to use,
but for it's age it still works really well.
(IMG:BC-221-AE_1.jpg)(IMG:BC-221-AE_2.jpg)(IMG:BC-221-AE_3.jpg)(IMG:BC-221-AE_4.jpg)(IMG:BC-221-AE_5.jpg)(IMG:BC-221-AE_6.jpg)(IMG:BC-221-AE_7.jpg)

Here's a YouTube video of a newer Frequency Meter,
plus the same model as mine (not my video) - (https://www.youtube.com/watch?v=9iZy41OfGHM)
and some more info and the User Manuel - (http://radionerds.com/index.php/BC-221) - (http://www.pa3esy.nl/military/us/meet/BC-221/BC221AE/html/bc221ae_set.html)