I have one of those. You are right, it could be called the Navy version of the BC-221.
They are also called LM frequency meters. Yours is probably an LM-10 thru LM-21 -- one of the later units.
You can find a schematic and info about power requirement in the Surplus Conversion Manual volume 2 which is available on the Web:
http://www.mines.uidaho.edu/~glowbugs/SurpConv.htmlEach unit has it's own calibration book which fits in the slot at the bottom of the meter.
The calibration manuals are different for each model and are have the serial number of the unit on them.
If your calibration manual is gone, don't despair as I once found a program on the Web that would print out the appropriate calibration data for your unit if you plug in the serial number.
Mine had the calibration book with it, so I didn't keep track of where that program was, but I suspect you can find it with google -- use "LM frequency meter" when searching as that's the more common name than the CKB number.
Dave