Conclusive evidence

Exploration Stage: Postal Dovecote
From: Mr. Black
Description: What Leon's pigeons have stolen may reveal the identity of the culprit. What we have so far is circumstantial evidence, but if it is an envelope that has not yet been sent and is unopened, could it reveal the identity of the culprit? Let's find out!
Completed: I recognize the sealing wax I just examined. The monogram on the sealing wax is an M, exactly the same one used by the suspect in the case I mentioned. This is important evidence, you know!