Why not ask them if they have a copy of the original report (that they mistakenly didn’t send to you) that they can potentially send you?
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If you are looking for the letter of the law, it would seem mechila b’taos eina mechila, (Especially as you didn’t discover the issue and continue to use the Megilla without complaining, since a claim does not start until the issue is actually discovered. So you’d have the right to claim from the other party even after a significant amount of time passed.)
On the other hand, the other party might claim Minhag HaMakom or Minhag HaSochrim absolves them after a signifcant amount of time.