This is a restored Feiler TS-3A Signal Tracer. They labeled it an Electronic “Stethoscope”. This signal tracer can be used for AF and RF signals. There is a tube detector built into the probe for RF signals. I did not have a schematic available, but I was able to restore the unit from the markings on the components in the unit and tracing the circuitry. This signal tracer works very well and it has been useful to trace signals in old radios.