My great-grandfather Arthur Rittel had an older brother Gus (August) who served in World War I. Recently I came across a history of his unit which included the roster with his name and the engagements he participated in.
The Story of the 139th Infantry follows the unit as they arrived in France and participated in a number of battles during 1918. The historian who compiled the book in 1920 recorded that Gus participated in the battles of Meuse-Argonne and Saint Mihiel as part of “I” Company.
August Rittel is listed on page 116 in the book which is available online here.
It’s possible that Gus appears in one of the many unlabeled photographs in the book, but most do not have any names listed below them.