Cutler Hammer Main Service Panel circa 1960.

Is anyone familiar with these main service panels from that time?

This is a 100 amp service with breakers.

I had some receptacles grounded by running the ground wires back to
panel. I had assumed these panels just had a neutral bus and
everything was put on that since there were not that many grounds.
Indeed, on the top is a horizontal bus that is basically a round piece
of copper (like a straw), and all the neutral/ground wires (incuding
main house ground) are wrapped around the bar--no screws.

I was surprised when I saw the electrician putting the new grounds on
another bus. This one is lower in the panel (also horizontal) and
looks like a flat peg board, but this had screws for connections. I
didn't get a good look, but it didn't look like there was any wires on
it.

Now that I am thinking about this, I am wondering if the two buses are
bonded.

In the late 60s or early 70s a central AC/Heater unit was added and
they used aluminum wire for it. So now I am wondering if it is more
likely that the bus bar with the screws was original or is it more
likely it was added when they did the Air conditioning wiring? I was
distracted so I am not sure, but I did see a piece of alumium wire
around there, but not sure if it was to the breaker or to that bar.

Anyway, has anyone seen this type of Cutler Hammer panel from 1960?
What would your take on this be? IF both buses were original, would
they have been bonded even though only the top one was used? If the
second was added, would the design be such that it would be bonded via
the case of the panel?

Obviously, I am new to this stuff, but I hope I was able to describe
it, even if I didn't use the correct terms.

--
John