Does anyone know the purpose of the coil spring that sits in the groov around the outside of brake drums? I have a 58 Chevy Stepside and only 1 of the drums has this spring but all drums have the groove for the spring to fit into.

Also, does anyone know a soucre for these springs? I have not been able to locate any in the catalogs.

Thanks

PS Sorry for this very minor question amoung all the great questions but I am new to restorations. :p