The Mystery Bean came from @Reznfingeez33, courtesy of the Mystery Seeds Give Away Thread. He sent his seeds in a chunk of corrugated plastic, and one of them made a run for it when I was loading them into my storage system. That bean got planted in the same pot as what would have been PoundCake #2, if PC2 had surfaced after a half-hearted germination. (She sent out a taproot, but it was pretty tiny.)
@Reznfingeez33 , would you care to venture a guess as to the identity of the Mystery Bean, judging by her vigor, structure, and rate of development?