"The last word may never be written about the BC-221 and LM frequency meters."    So writes Charles Landahl in the May 1971 issue of 73 Magazine.  More was certainly written 37 years later in the excellent 5-part series by Marc Ellis (1933-2020) in the 2008 May to September issues of Monitoring Times.    The articles can be found at https://worldradiohistory.com  .

I acquired five (!) BC-221 units at various hamfests over 20 years.  In retirement, I had time to examine them for functionality and differences.

Two of the BC-221 units came with homebrew power supplies.  They looked too risky to try, so a first task was to build a supply for the 6 volt filaments and 135 volt plate voltage.  See the presentation here . 

The (long) presentation below covers differences between models and oscilloscope measured waveforms.

If I can choose a BC-221 unit for experimenting,  one might have its tubes replaced with transistors, as in the Charles Landahl article.

"Checking the Frequency of a BC-779-A Receiver at the signal repair section of General Stilwell's headquarters, Burma."

Photo from  p. 412, The Signal Corps: The Outcome (Mid-1943 Through 1945), George Raynor Thompson and Dixie R. Harris.