MLA

Beinecke, Edwin J. (Edwin John), 1886-1970. "RLS/C". Box 38, folder 850. 1881, undated. https://collections.library.yale.edu/catalog/10595528.

APA, 6th edition

Beinecke, Edwin J. (Edwin John), 1886-1970. (["Box 38, folder 850"])(1881, undated)["RLS/C"]. https://collections.library.yale.edu/catalog/10595528.