PBY-5A exhaust modification?
Thanks for the suggestions in the comments.

Using the term “heat exchanger” I was able to find an Aircraft Modelling forum that gave the answer I was looking for. 

Interpreting wartime photography: Catalina in Ceylon

It is indeed a heat exchanger system for leading edge anti-icing. The tail appears to have a separate anti-ice system that burns fuel for heat. This thermal anti-icing was introduced on late model PBY-5A. It probably was an optional system.