From the 1962 biography of Adolph by the Stewarts:

“On the second of March , 1881, he and his daughter Emma rode out past the western edge of the city… they found a small white cottage they had never seen before. … The house and its plot of about one and one-half acres belonged to Samuel Tetlow, proprietor of the Bella Union, whose business had declined in recent years. The two Sutros found Mr. Tetlow at home and Adolph immediately entered into an agreement to purchase the house and land for fifteen thousand dollars.” (p 173-174)

And then on p 226:
“His [Adolph’s] discovery of Sutro Heights is based on various accounts including a newspaper interview of Emma in the San Francisco Chronicle, Nov. 19, 1933, p. 6. The date in the latter has been corrected to 1881 win accordance with the agreement between A. Sutro and S. Tetlow dated March 2, 1881, in Adolph Sutro, Safe Book, Agreements, p. 51 in ‘Stanford Univ. MSS’.”

So the Stewarts used Emma’s interview, but discounted her 1879 date and instead used the March 1881 date from a document apparently at Stanford.