Where is the Masterwork Chest + Key reward on the Legends of Bilgewater missions list?

https://na.leagueoflegends.com/en/news/game-updates/special-event/learn-more-curse-drowned-event

On the list of rewards for completing all the Legends of Bilgewater missions, it mentions "1 Masterwork Chest & Key." Every other reward on that list is listed as a reward for completing various missions, but none of the missions give a Masterwork Chest.

Is the Masterwork Chest a special reward for completing all of the missions, is it not a reward at all, or was one of the missions left off the list?