Aluminum right? The sheathing looks that of aluminum but there was a hint of gold on the wires making me second guess it. It was in a 1950s home with 4 separate panels. This Square D panel was in the garage and may have been added later as the other three were Cutler Hammer panels and no signs of AL in those. Thanks.
I’ve never seen Al that colour before!! Could it be 2 ages of copper with black corrosion on the older wires? Have a look at the Al service neutral at the top of the picture…it’s nice and clean.