In the interest of the integrity of scholarship, let us recall that besides Seminar 5 already discussed, Lacan addressed specifically Freud’s notion of Nachträglichkeit many times. Often Lacan refers to this notion using, as Jonathan House notes, the adverbial form nachträglich . Note that in 1972 this notion is not used as retroactivity or retrospective modification and very much like Laplanche, Lacan gives to this notion a foundational status in psychoanalysis.
For those curious to know where and when, I quote the specifics:
Seminar 1, session of 2 June 1954 (reach the child by way of the adult)
Seminar 15, session of 22 November 1967 (nachträglich and symptomatic action)
Seminar 16, session of 7 May 1969 (nachträglich and the seminar)
Seminar 16, session of 25 June 1969 (nachträglich and unary trait and S1)
Seminar 19, session of 8 March 1972 (elaborated as different from retroactivity or retrospective modification)
Seminar 19, session of 10 May 1972 (nachträglich and revelation)