A replica of the Front Door key to the Guild Hall of the Guild of Monsters in Mechanicsburg.
From this webcomic:
Girl Genius (girlgeniusonline.com)
For the version from the souvenir shop, see this product:
https://www.shapeways.com/product/TUS93HEJT/monster-hall-key-souvenir
Unlike the original, this replica is 97% guranteed to remain in your posession.