Faithful readers of the Legion of Superheroes must have been confused at this sequence, which appeared in Adventure #317:
There are certainly still some problems with this speculation. For example, the story does not end with the Time Trapper in the future, creating the Iron Curtain of Time. But this objection is easily overcome; Weisinger simply had the ending of the story rewritten because now it took place after the events in #317, instead of before. Note as well that the story in Adventure #338 did not explain what secret the Time Trapper was supposedly concealing from the Legion in the future.
So my best guess is that the Time Trapper story that was supposed to be published before Adventure #317 was in fact the story that ended up being published in Adventure #338, with some changes.
Incidentally, the Time Trapper himself may have been based on the Time Master, a similar character that appeared in Wonder Woman #101: