Anzieu (1975/1986, p. 538), I think, also gets the credit for first fully appreciating the link between the "Dora" pseudonym and another Breuer daughter--18-year-old Dora, born during the treatment of Anna O.
Hirschmüller (1978/1989, p. 314), in his biography of Josef Breuer, has reprinted a letter written to Fliess by Breuer's daughter Dora (then 12), concerning her recent nasal surgery:
Thursday, March 19, 1895
Papa has told me of your inquiry as to how things are with my nose. I prefer to write to you myself, since Papa can have no idea how happy I am that you have done this for me. I go around with my mouth shut, I don't wheeze any more, and the buzzing and occasional deafness in my ears has ceased. I am so happy, and sometimes I think how awfully stupid I must have looked always going around with my mouth open. I only hope that Selma B. is just as fortunate when she gets back from Berlin. Thank you very, very much, yours,
Hirschmüller suggests (p. 416, n. 273) that Dora Breuer's operation may have been performed on the same Vienna visit (February, 1895) during which Fliess operated on Eckstein.
Hirschmüller, A. (1989). The life and work of Josef Breuer: Physiology and psychoanalysis. New York: New York Unuiversity Press. (Original work published 1978)