One thing that is interesting, I think, is Volume 2 which features in one volume the story of Cobalt. In Volume Two of this collection, Cobalt's origin story is featured along with the episode which features his "death", as well all of his appearances in between. They use the original episode title as it appeared in Shonen manga.
Here the revised pages covering up Cobalt/Jetto's death. They are shown only to illustrate the difference in how the panels were changed. You can see for yourself how Atom replaced Cobalt and how Cobalt's death scenes were removed.
MIDORO SWAMP -PRE EPILOGUE.jpg (132.31 KiB) Viewed 2894 times
MIDORO SWAMP-EPILOGUE.jpg (101.16 KiB) Viewed 2894 times
In the early Manga it's hard to tell the difference between Atom and Cobalt, Tezuka drew them almost as identical twins. Cobalt also had most of Atom's abilities, he was just as strong, and had leg jets enabling him to fly. In later Manga, and in the Anime, Cobalt was a bit taller, his hair spikes were thinner, his nose was a bit larger than Atom's, and he lost his leg jets. IIRC, Cobalt also didn't have any eye lashes.