The closed area to the left is where the utility feeders enter. This area is sealed, and not accessible to us, as a rule. If there is a neutral buss, it will typically extend through the partition, and have places to terminate neutrals in the area that I have opened- just as this area has points to attach main breakers for each service.
Not bad, Norcal....my pic doesn't show it well, but that "buss" below the breakers is not tied into the utiulty at any point, and has only uninsulated #4 ground wires connected to it. (Hard to see with all the dried river mud!)