I don't have reliable information about an artwork titled "Untitled (Mobile)" by Gertrude Stein in my training data. While Gertrude Stein was a renowned writer and art collector in early twentieth century Paris and New York, she is not known as a visual artist who created sculptures or kinetic works with these materials. It's possible this is a lesser known or recently attributed work, or there may be an error in the attribution. I'd recommend verifying the artist's name and provenance through a museum database or archival source.