[Mondial]

Looks like the 7 pin socket might have been for an external crystal calibrator. There is no way that a tube plugged in to that socket. Perhaps a previous owner built a separate calibrator box and connected it by a cable to the 7 pin socket.

It appears the socket provided 6.3V and B+ for the calibrator along with a connection to the antenna circuit and provision for the switch to turn it on and off. That's exactly what would be needed to connect an external calibrator.