Event: Petyr Baelish (Littlefinger) - Secret Discovery (Political Wins)

Episode 5: Eastwatch at 51:24
Littlefinger acquires the only copy of a raven letter written by Sansa while she was in King's Landing, when she was compelled by Cersei to ask Robb Stark to come south and bend the knee.

Maester Wolkan: "I found it, my Lord. It took some searching. Maester Luwin's archives are very thorough."
Petyr: "You're sure this is the only copy in Winterfell?
Wolkan: "Yes, my Lord."
Petyr: "Lady Stark thanks you for your service."
(Secret Discovery)