Old analog oscilloscope (Advance - Gould OS 240) front panel output
Just to answer the question... after finding, purchasing and reading the manual. ...According to the manufacturer: "This output provides a positive going sq. wave of approximately 20V amplitude from a source impedance of 15kOhm. Its freq. is dependant on the setting of time/div and is suitable for probe compensation.
If anyone ever cares or needs the manual I can send the pdf.
Thanks