MLA
Gates, Howard C. An Advertisement for the Knox County Fair in Mt. Vernon Ohio, from Sept. 12-15, 1911 : with a Young African American Boy and a Horse. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 70. https://digital.library.emory.edu/purl/443nvx0k9p-cor.
APA, 6th edition
Gates, H. C. An advertisement for the Knox County Fair in Mt. Vernon Ohio, from Sept. 12-15, 1911 : with a young African American boy and a horse (Box 70) [Postcards]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/443nvx0k9p-cor.
Chicago
Gates, Howard C. “An Advertisement for the Knox County Fair in Mt. Vernon Ohio, from Sept. 12-15, 1911 : with a Young African American Boy and a Horse.” Postcards. Robert Langmuir African American Photograph Collection, n.d. Box 70. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/443nvx0k9p-cor.