It's interesting how there is a 'XTAL CHECK' position but no obvious input for a crystal. Is there a crystal inside the generator?
I think 'Phones' is for a 1/4" headset. Some generators had a feature where you could listen to the beating between a crystal frequency standard and the RF output of the generator as a way to check calibration.
The Taylor 62A MKII is a similar instrument of the period. It was probably a competitor with the Advance 63A you have.
https://www.google.com/url?sa=t&rct=...kcyqjGCSXpsrac It might be worth having a look at how the Taylor works as the operating principles could be applied to your Advance.
I'm not sure about the mains setting. Is it just an unswept FM wave? With a 50Hz tone?
Food for thought.