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It's a KS-19355 L8 Adjuster, used to tune the ferrite coils on a 25- or 35-type Touch Tone® dial. There's a coil for the high tones and a coil for the low tones. They are the round gray things on the back of the dial. See BSP 074-202-121.

You tune them by looking at a frequency meter, or, in the field, by using a known good dial and "beating" them.


[Linked Image from beatriceco.com]


Arthur P. Bloom
"30 years of faithful service...15 years on hold"