Aha, so that's where this was coming from... Thank you for pointing that out! :)

Thought I must have missed a free key somewhere, like all the random f2p game dlc giveaways you see around here... But checking my Humble E3 2017 purchase, somehow I had placed this key in my "used keys" pile, without actually using it. Oops. :) Finally redeemed it now. :)

Thank you for giving this away, and good luck everyone else! :)