[C210N]

I used to have one of these babies... cool thing was that, when connected, it could "listen" to the phone line without opening the microphone circuit.

[anton]

Me too. And, when you wired in a capacitor from a tube radio they could not hear a click when you connected the wires and tapped the line.

[ptsal]

....back in the days of closely trimmed hair with white-walls, Brooks Brothers suits, and thin black ties.

Thanks for the image.