What does the PBY Catalina's APU look like?
The link you provided is for a Canadian PBY-5A which were called "Canso" in Canada. I flew these aircraft in water bombing operations during the early 1980's.  

In your link it describes the 1955 installation of a 28v Homelite APU. It was installed on the left (port) side of the aircraft in the "living quarters" compartment behind the landing gear. The Homelite APU was powered by a 1 cylinder 2 stroke engine, and were often referred to as a "Putt Putt".  They were only used if the Main batteries were weak or discharged. 



This is a photo of a Homelite APU from a B-17. They were also used on B-24s and other large WWII aircraft.  It is nearly identical to the ones we used in the civillian PBYs that I flew.


The original military PBY-5A APU installation looks a little different. It appears to be located on the starboard side in the central compartment near the landing gear. It would have been removed when the water bomb tanks were installed.