Colonel Mann [and] Colonel L. Clem - Glass negative photogrpah. Public domain.

Photo possibly shows Colonel John M. Clem , the "Drummer boy of Chickamaugua" (1851-1937) and William d'Alton Mann (1839-1920) a Civil War soldier, businessman, and newspaper publisher, at the Gettysburg Reunion (the Great Reunion) of July 1913, which commemorated the 50th anniversary of the Battle of Gettysburg. (Source: Flickr Commons project, 2009)