MLA
Tillinghast, Edwin L. Envelope for Letter from Edwin L. Tillinghast Sent to His Sister, Cornelia E. Tillinghast (Gillisonville, South Carolina). 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 6, Folder 55. https://digital.library.emory.edu/purl/573bzkh1zb-cor.
APA, 6th edition
Tillinghast, E. L. (1861). Envelope for letter from Edwin L. Tillinghast sent to his sister, Cornelia E. Tillinghast (Gillisonville, South Carolina) (Box 6, Folder 55) [Envelopes]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/573bzkh1zb-cor.
Chicago
Tillinghast, Edwin L. “Envelope for Letter from Edwin L. Tillinghast Sent to His Sister, Cornelia E. Tillinghast (Gillisonville, South Carolina).” Envelopes. Civil War Collection, 1861. Box 6, Folder 55. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/573bzkh1zb-cor.