Letter with no Address

Exploration Stage: Post Office
From: Mr. Black
Description: I admit, you found a new clue in the search for the valuable cargo. But the monument of the mailman has not yet led us to our goal. What a pity it cannot be interrogated. Where did he carry this letter that once contained the plate? Find the answer.
Completed: This book Holder is made in memory of the earthquake. The City was divided into lower and upper parts, with the Rift - a deep gorge - between them. What does the mailman have to do with this?