What was the dividing event? I can't see any noteworthy change from one cover to the next.
The dividing line is arbitrary, as the G.A ends as of September 1956, and the S.A begins as of October 1956. The Silver Age of comics begins with the publication of Showcase #4 in October 56, when the new Flash premiered.
In my opinion a better place to start, just for the Superman title would be #117 "The Man with the Zero Eyes", the first cover by Swan, or perhaps #121, "The Bride of Futureman" (this cover has the S.A. look) (thumbs u
If your interested I have the back issue of Comic Book Markeplace, where there is an article titled something like, "Carbon Dating the Silver Age of Comics". This might be only for DC books, if I recall correctly. I could check for their analysis on the Superman title.