Brake Drum Spring

It may be the shape, thickness, or alloy of the drum. I have seen them on other cars, but don't ask me "which". IT HAS BEEN a very long time LOLOL

Turns out Corvair used them............

Spring around the outside of the drum.

C2 Corvettes..........

Purpose of Springs AROUND Brake Drums - CorvetteForum - Chevrolet Corvette Forum Discussion

Ponch GTO..............

Mastering GTO Restorations: Brake Guide