Seems that in 1968 Kac gave a seminar where the first idea of the Replica Trick appeared. It was the Trodheim Theoretical Physics Seminar. Do someone has a copy of this work or is it lost?
why not check at trondheim's libraries' database?
if this seminar took place there, then probably they have a copy of it.
although it seems pretty tough to get them send you a copy of it, although not impossible. (-: