[Boatanchors] How did they "pinpoint" frequencies way back when?

[Robert Nickels]

> They are a heterodyne type meter, that used
> a small  ring-leaf booklet, that was supplied by the manufacturer with
> calibrated dial readings, corresponding to frequency output. Each booklet
> was dedicated to its unique unit.

Which brings up an equally interesting question: How did the manufacturers
calibrate the frequency meters in the first place?  Obviously this was a
manual process that had to be done individually for each unit, and I suspect
they used the same method of calibration, only using a secondary standard
that was built to even higher levels of accuracy and stability back in those
days before digital counters existed.

I no longer have one but as I recall there were a lot of cal points in those
books!  I'll bet some form of interpolation was used between a number of
fixed points - anyone know for sure?

73, Bob W9RAN