[Radio Wrangler]

The oscillator and the meter have to remain in tune or else the meter result will fall from the peak and this will affect calibration of the Gm ranges

The tuned nature of the Gm meter, I suspect is a ploy to reduce its sensitivity to mains harmonics. It might be best if it were tuned to a frequency which is not an integer multiple of the local mains frequency, and that the oscillator is tuned to match.

David