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Back Issue #1 and Back Issue #2 (aka Variant 1 and Variant 2) have 2015 rank up gems to rank any 5-star Rank 3 champions to Rank 4. Exploring each chapter gives you one 2015 rank-up gem.
Recently Kabam announced Back Issue #3 (Variant 3) that is scheduled to available this month after the app receives the new update. However, Players started talking they are tired of those 2015 champs as there aren’t many good champs to rank.
So Kabam Mike has already clarified that Variant 3 will include 2015+2016 rank up gems. That means you will be able to upgrade any 2015 or 2016 champs from Rank 3 to Rank 4.
If you want to know what champions were released in 2015 and 2016 you can see the list of champions by their release date here.