Superman #146 (May 1961) has a great re-telling of Superman’s origin, deftly weaving in all sorts of facts and notes of the Superman mythos up to this point, but I found its second feature “Superman’s Greatest Feats!”. In it, we see Superman convinced by Lori Lemaris to try and save Atlantis from being sunk. Superman tells her he has tried to change things in the past (including saving Lincoln from being shot) and has always failed, but he tries again. This time, amazingly, he is able to save Atlantis, Lincoln and several other historic figures.
Superman then has a brainstorm to save Kryptonians from Krypton’s destruction. He travels into the past, constructs and sends a series of rockets to Krypton with a note to Jor-El to have Kryptonians flee the planet. And this also works!
By this time, Superman’s astonishment is only matched by the readers of the comic book, who know that these feats can’t be sustained, changing Earth’s past forever. Superman observes Zor-El, Lyla Lerrol, Jor-El, Lara and baby Kal-El getting off the rocketship on Earth.
Streaking through the time barrier to the present, Superman inspects the library and learns that none of his feats are recorded. Lincoln was still assassinated, Atlantis was not preserved, and a fleet of Kryptonians didn’t land on Earth. Hm, what gives?
Once more Superman flies into the time barrier and this time notices that there is a parallel time barrier that bridges universes. It turns out that Superman had entered this parallel universe, where the laws of physics were such that he is able to change the past, and that is the past that he affected.
Superman confirms that all the library books have recorded the events he changed in this universe, but abruptly detects that the twin universe is “quivering”. He quickly escapes the twin universe to arrive back at his own boring one with the unaffected timeline.
He flies back to Lori and tells her what has happened and how he could not stay in that universe since it could de-stabilizing either or both universes. However, it does leave the door open for future tales of this strange twin universe! Also, I have no idea why Lori is smiling at Superman with that creepy, half-lidded gaze…
While this isn’t exactly the standard DC multiverse concept (which is just a few months away from being official), it is pretty darn close. The fact that there are suddenly bridges that Superman can encounter when breaching the time barrier is definitely interesting.