MLA
Envelope Addressed to Sarah L. Walden (Providence, Rhode Island) by Edward Everett (Boston, Massachusetts). 1862. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 19, Folder 21. https://digital.library.emory.edu/purl/972p5hqd40-cor.
APA, 6th edition
Envelope addressed to Sarah L. Walden (Providence, Rhode Island) by Edward Everett (Boston, Massachusetts) (Box 19, Folder 21). (1862). [Envelopes]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/972p5hqd40-cor.
Chicago
“Envelope Addressed to Sarah L. Walden (Providence, Rhode Island) by Edward Everett (Boston, Massachusetts).” Envelopes. Civil War Collection, 1862. Box 19, Folder 21. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/972p5hqd40-cor.