Old electrical panels?

I have run into a few very old electrical panels, and though I have plenty of reference material for the newer panels and wiring, I’m having a hard time finding material on panels that are 40 years or older. Can anyone recommend a book or website that can help?

Electrical Inspection of Existing Dwellings

By Douglas Hansen, Redwood Kardon & Mike Casey

Those are the same guys that create the ITA Handbooks as well and Code Check.

Thanks