MLA

Speck, William A. (William Alfred), 1864-1928. ALS : Weimar, to Christian Gottlob von Voigt. Box 9, folder 301. 1811 May 4. https://collections.library.yale.edu/catalog/2005343.

APA, 6th edition

Speck, William A. (William Alfred), 1864-1928. (["Box 9, folder 301"])(1811 May 4)[ALS : Weimar, to Christian Gottlob von Voigt]. https://collections.library.yale.edu/catalog/2005343.