Envelope X Spoilers, end-game spoilers:

So our party opened Envelope X a long time ago - months ago, in fact, it was the very first retirement reward we unlocked. While we've found a couple of the clues to solve the puzzle, we're now rapidly approaching the end of the campaign (Scenario 51) and I'm not sure our group will stay motivated to clear random missions after unlocking all the classes and beating the canonical final boss.

Our group has always loved the cryptograms and often made decisions based on which missions might hold more - descriptions of "runic language" were often interpreted as hints that the mission would have more Envelope X puzzles, but they almost never did. Also, if Envelope X rewards something that increases character power somehow, it would be nice to get it before we're completely done with the game.

So, to save us the trouble, is there some way to know which scenarios give hints that make solving Envelope X possible? We already cleared Scenario #76, Harrower Hive, and got the hint there. We cleared Scenario #15, and got the hint there.

Most of my attempts at googling yielded more spoilers than what I would have liked to see. I'd really like it if commenters could avoid spoiling Envelope X altogether - all I'm hoping for is to know where to start looking, because we still have ~30 uncompleted scenarios, but I doubt the group will stick around grinding random missions for that long.