Iris's postcard was evidence in the trial of Soseki Natsume for the stabbing of Olive Green.
Iris Wilson wrote in the postcard, "Tell the gentleman in black whatever he wants to know. I trust that won't be a problem?" She gave the card to Ryunosuke Naruhodo to deliver to Tobias Gregson, as well as a five-shilling coin meant to pay his "appearance fee" in a The Adventures of Herlock Sholmes story. To Naruhodo's surprise, Gregson did in fact tell him what he needed to know, namely that Natsume lived in the Garrideb house across the road from where the incident had been claimed to have occurred.