User blog comment:Omega natsu2/Chapter 371: Tartarus Arc, Part 2: Song Of The Heavenly Dragon, Predictions./@comment-86.70.179.7-20140210084523/@comment-4489126-20140210135922

That key looks like the thing they used for Edolas.

But if the thing about the Dragon key is right, I can see Hiro saying that it was one of the keys (supposing he gave her that for the past 7 years) Jude gave her for birthday. But then again, why not use it against a Dragon when they attacked. :/