Navajo “coppered” silver cuff bracelet ??!!

[Ag999]

A Leroy James Navajo cuff bracelet from the 1970s (?). Several points verify that it is his work: the signature and “sterling” stamp, type of turquoise used, and overall style (including the two “buttons”). But I have never seen “coppered” silver before. Has anyone? By him or by any Navajo silver maker? There are several areas where the darkened silver shows through the thin coppery plating.
Any thoughts, comments?