On Wednesday, January 14, 2015 at 1:45:04 PM UTC-8, John Ferguson wrote:
There are a number of ADs for the Pawnee, one specifically for corrosion
issues in the rear fuselage structure. You might just be seeing these
problems coming to light.

Adding to what Fred said re corrosive and hygroscopic chemicals in their former life, many Pawnees live outside where they are often subject to large temperature and humidity changes. Any condensation within the tubing runs downhill. The tail post cluster is a likely first victim.

The horizontal stab spars have an AD too.