Mont Saint Michel Brooch store from Normandy

This vintage (possibly antique) Mont St Michel brooch is of a gold-colored metal with pearlescent background. I was told the metal is gold-plated, but store there are no visible marks denoting this.

The brooch has a working trombone clasp, which dates it to the late 1800s, early 1900s, so I am dating it conservatively as being circa 1920s.

Dimensions are 1 ⅛" across.