PBY-5A exhaust modification? What is the function of these “tubes” that cover the outboard exhaust stacks on some PBY-5A and PBY-6A aircraft?
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.