APA (7th ed.) Citation

Seward, W. H., Barton, W. E., & William E. Barton Collection of Lincolniana. (1860). Autograph letter signed: N.p., to Horatio King, n.p.

Chicago Style (17th ed.) Citation

Seward, William H., William E. Barton, and William E. Barton Collection of Lincolniana. Autograph Letter Signed: N.p., to Horatio King, N.p. 1860.

MLA (8th ed.) Citation

Seward, William H., et al. Autograph Letter Signed: N.p., to Horatio King, N.p. 1860.

Warning: These citations may not always be 100% accurate.