PBY Catalina International Association
Cutaway PBY Project
This is the "tower" in the cabane strut where the flight engineer monitors engine and fuel guages, operates mixture controls, starts engines, manages cowl flaps, wing tip floats, and acts as observer through the side windows.

[EM: I've been chided that the side windows were for observing the engines, not looking for the enemy. While the PBY veteran who wrote this doesn't say that the side windows were used to observe the enemy, I could certainly believe that the engineer had time to add his eyes to the patrol effort.]