MLA
Pierce, George F. (George Foster), 1811-1884. Envelope sent by George F. Pierce (Sparta, Georgia) to Robert Paine (Aberdeen, Mississippi) - June 25, [no year]. June 25, year unknown. Robert Paine Papers, 1828-1882. Pitts Theology Library. https://digital.library.emory.edu/purl/53976hdsfr-cor.
APA, 6th edition
Pierce, George F. (George Foster), 1811-1884. (June 25, year unknown)[Envelope sent by George F. Pierce (Sparta, Georgia) to Robert Paine (Aberdeen, Mississippi) - June 25, [no year]]. Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/53976hdsfr-cor.
Chicago
Pierce, George F. (George Foster), 1811-1884, Envelope sent by George F. Pierce (Sparta, Georgia) to Robert Paine (Aberdeen, Mississippi) - June 25, [no year], June 25, year unknown, Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/53976hdsfr-cor.