It was attached to the repeater of the sensor and used to supply power from the constant voltage power supply to the sensor. It is a nostalgic feeling, but things are firm and no problem. Temporary connection such as test equipment is still useful.
It is active as a takeout terminal of DC. I was relieved that the plug of 50 years ago was compatible and could be used comfortably. It was disappointing that the length was 5 mm shorter than the terminal knob of 50 years ago and the operability became worse.