George Gruhn explains "From the early 1900's through early '20s, mandolin orchestras were extremely popular, in part because Gibson instruments were sold primarily through music teacher/Gibson agents, rather than music stores. Gibson established a network of agents who organized mandolin orchestras, recruited students, and sold Gibson instruments on payment plans, which greatly increased sales and meant instruments could be sold at higher prices. The company even sent posters to agents so they could hang them around their town promoting concerts and other events where they could recruit students. (and potential Gibson instrument buyers) In addition to publishing instruction methods for students and manuals for teachers, Gibson catalogs from the 1910s and '20s are filled with photos of mandolin groups from all over the US. Though we don't believe your photo (see below) is such an example, a photo showing the same instructor/Gibson agent- Joseph Ivers appeared in Gibson's 1921 catalog.
Here's the picture. (I'm working on acquiring a higher resolution copy)
Thanks to the Vintage Guitar Magazine reader who found the picture and sent it to the magazine. Thanks also to George Gruhn for taking the time to answer the reader's question. Special thanks to Vintage Guitar Magazine editor Ward Meeker for sending me the picture.
Here's a cropped high resolution version.