Here is another first that I have found: In DC comics, there was this concept of multiverses and twice each year the JLA-stories would focused on a comingling of the residents of an Earth-One and an Earth-Two. It was said that Earth-one was our world. One reason for this was because Earth-Two’s time line was advanced from our world’s time by about 20 years. For example, in 1967 the Batman television show was current with the Batman comics, Detective comics, and every place else except whenever the Justice League would encounter the other universe containing Earth-Two (which occurred usually every summer from 1963 to 1986).
In that other universe, Batman was retired and Bruce Wayne was now police commissioner of Gotham City. Also, Dick Grayson/Robin was no longer “the boy wonder”; he was now a young adult. So, in the August 1967 issue #55 of JLA, a twenty-something Robin makes his appearance as the newest member of the Justice Society of America. At this point, there were no name changes for Robin, it was mainly a change in costume.
The time difference between Earth-One and Earth-Two served more than one purpose. First of all the elder characters of Earth-Two were from the Golden Age of Comics (the 1940s to the early 1950s). Yet instead of Earth-Two being 20 years behind, they were placed 20 years in the future. This allowed the writers space to invent new back-stories – a technique that is now called “Retroactive Continuity” or “retcon”. Secondly, it allowed the writers room to experiment with “alternate histories”. For example, having Batman marry Catwoman.