Michael Black wrote:

Nowadays, it's a different matter, since anyone with a frequency
counter can live without a calibration book, or at least make
up a calibration book far easier than sixty years ago. They likely
have access to a very good frequency standard too.

Precisely. The thing is, with a good counter you don't need a BC-221
any longer.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."