What you seem to have is six standard cells in a temperature controlled enclosure.

Possibly the terminals are something to do with checking the temperature as you need to know that to know the exact voltage.

These would originally have been used with a potentiometer and compared one with the other to establish the most accurate voltage. You must not take current from them but the 10m input impedance of a digital meter will not hurt them.

Unless you collect curiosities I really can't think what you are going to use them for except to check your digital meter which seems to be spot on.

Hang on a minute, I think your GA terminals and the toggle switch may be part of the comparison process. I think GA means galvanometer. A highly sensitive moving coil galvanometer ( microammeter) that would have been used to detect slight variations between cells. You might even have a basic potentiometer circuit built into the thing.

They will be Weston standard cells, probably you can Google something about them.