MLA

Thayer, Scofield, 1889-1982. Thayer, Florence: letters from Scofield Thayer. Box 80, folder 2071. 1925 June-July, undated. https://collections.library.yale.edu/catalog/17142348.

APA, 6th edition

Thayer, Scofield, 1889-1982. (["Box 80, folder 2071"])(1925 June-July, undated)[Thayer, Florence: letters from Scofield Thayer]. https://collections.library.yale.edu/catalog/17142348.