Where do I find the First Cipher?

Where do I find the First Cipher in Bioshock: Infinite? I've looked just about everywhere in this map I can think of and can't find it. Anyone got any ideas?

You can achieve it via an optional mission at the Soldier's Field/Hall of Heroes.

Upon your initial visit to Sky Road, you’ll be bombarded by enemies. Take them out and then head into the Fellow Traveler, the local bar.

If you examine the cannon picture where the dead soldier is lying carefully, you will see a circle around the bore of the cannon. That's our clue.

Go outside and to the building across that says "Tickets" and then look into the bore of the cannon to find the codebook.

Elizabeth will decipher the code as "Tip the Hat to the Vox", which indicates that you should talk (interact) with the hat in the bathroom next to you.

This will open a secret door to a room with nice loot to collect.