Pretty sure they're brass.

A guy I know has an old Eagle Brand accordion that's decked out in brass. It has brass corners and hardware, brass buttons, brass corners on all of the bellow folds, and even the half dowel rods for the flaps are covered in a thin brass sheet instead of the gold paint they were using on the Monarchs, Sterlings, and Globes.

John

First accordion that I built, I made triangle shaped rods out of Copper. It took some doing and some time. This shape is the original shape of the Sterling and Monarch instruments.