MLA
Miller, C. A. Union Patriotic Envelope - Variety of Patriotic and Military Imagry with the Description, "Union". Published by C. A. Miller, 25 Ann St., New York City. C. A. Miller, 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 21, Folder 7. https://digital.library.emory.edu/purl/56341ns2zz-cor.
APA, 6th edition
Miller, C. A. (1861). Union patriotic envelope - Variety of patriotic and military imagry with the description, "Union". Published by C. A. Miller, 25 Ann St., New York City (Box 21, Folder 7) [Envelopes]. C. A. Miller; Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/56341ns2zz-cor.
Chicago
Miller, C. A. “Union Patriotic Envelope - Variety of Patriotic and Military Imagry with the Description, ‘Union’. Published by C. A. Miller, 25 Ann St., New York City.” Envelopes. Civil War Collection. New York City, NY: C. A. Miller, 1861. Box 21, Folder 7. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/56341ns2zz-cor.