MLA
Magee, John L. Union Patriotic Envelope with Robert Patterson Cartoon - "Major-General Patterson", Published by John L. Magee, Philadelphia, Pennsylvania. J. Magee, 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 21, Folder 6. https://digital.library.emory.edu/purl/556931zdxq-cor.
APA, 6th edition
Magee, J. L. (1861). Union patriotic envelope with Robert Patterson cartoon - "Major-General Patterson", published by John L. Magee, Philadelphia, Pennsylvania (Box 21, Folder 6) [Envelopes]. J. Magee; Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/556931zdxq-cor.
Chicago
Magee, John L. “Union Patriotic Envelope with Robert Patterson Cartoon - ‘Major-General Patterson’, Published by John L. Magee, Philadelphia, Pennsylvania.” Envelopes. Civil War Collection. J. Magee, 1861. Box 21, Folder 6. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/556931zdxq-cor.