BioShock Infinite: Soldier's Field Find the Cipher

This is an optional mission objective that you get at Soldier's Field/Hall of Heroes

In the restroom of a bar, you find a code written in blood. A soldier lays dead near the code, and he has a picture of a cannon with a circle on it:

Hint

Elizabeth can decrypt Ciphers that hide secret Vox Populi compartments. Find the Cipher and its corresponding code book to uncover hidden treasure.

Code Book Location Clue

The pic of the cannon is the clue, and the red circle around the tip tells you that the Code Book will be found there

Walkthrough

Go outside and to the building across that says "Tickets":

Code Book Location: Look in the hole of the cannon to find the codebook:

Elizabeth will decipher the code as "Tip the Hat to the Vox"

Go to the adjacent bathroom and interact with the hat:

This will open a secret door:

Loot the room as it has lots of stuff in it