User blog comment:B214/Volume 22 spoilers/@comment-82.48.158.172-20161109231053/@comment-9595671-20161111064902

He could have also had a mutation Bishop or Rook, that's probably the one big downside to trading a piece that hasn't been used, you may not now unless you can get someone powerful to test it on.

I mean he could have had Le Fay help him there, but I think most of his pieces would have been enough because of how strong he's become, and I can't think of anyone Issei could have gotten on short notice who's strong enough to help him.

If it was one of his Bishops then it's gone, if not then he's likely still got it, all I know is there hasn't been any instance where a Devil's gotten two mutation pieces that we know of if at all.