This is a Cutler Hammer split bus panel. I couldn’t see a panel bond, and the sticker on the panel was not readable. I suspect it is one of the two screws at the bottom of the bus. Can anyone confirm?
This split bus panel has two lower sections each fed by a 60 amp breaker in the upper section. I have only seen this once before, and that was last week. I did see the double tapped neutrals.