MLA

Speck, William A. (William Alfred), 1864-1928. LS : Weimar, to Christian Ernst Friedrich Weller. Box 9, folder 303. 1826 Mar 24. https://collections.library.yale.edu/catalog/2005345.

APA, 6th edition

Speck, William A. (William Alfred), 1864-1928. (["Box 9, folder 303"])(1826 Mar 24)[LS : Weimar, to Christian Ernst Friedrich Weller]. https://collections.library.yale.edu/catalog/2005345.