Flatten the Earth Society is a card in The Works. The card details are Mad, Giant, and Pests.
The card is not sepia-toned. It's possible that we have seen the Society in the Siege of Mechanicsburg.[mad theory] Here, it, or a member, is depicted as a large clank pushing a roller while being directed by a human on its head.
The instructions are the same as for Agatha's First Clank.