Internal A-Pillar Rot, What is This Panel?

[FitzK25]

I'm in the process of repairing the door hinges and patching the bottom of the kick panel on the driver's side of my 75 GMC. The amount of heavy scale raining down from inside the pillar had me concerned, so I started looking around. There appears to be a baffle of some sort above the upper hinge mount, inside the pillar, that has rotted out. There is no rust on the exterior of the pillar, on the inside or outside, and I've already had the window out of this truck (Window frame was mint).

What is this baffle? Is it structural? How would you even access it without peeling apart the entire A-Pillar? The only thing I can find that shows it is this photo I'm attaching of an entire door surround frame patch panel, it appears to the the little baffle I've circled in red.

 

[FitzK25]

Followup - I cut an access window into the pillar, and was met with this mess - After vacuuming everything out, that internal panel is pretty much completely missing, and a little of the top of the upper kick panel as well. I see no way to repair/replace this without completely de-laminating all four layers of the pillar. No idea what caused this as there is no mouse nest or other buildup of any sort that would have rotted it out from the inside. I am tempting to clean everything up, paint it, ignore the missing metal, and weld the pillar back up. Any ideas/thoughts?

 

[bucket]

Well that's why you don't go pokin', lookin' for things you don't wanna see! Lol.

Honestly, I'd just clean it up, coat the inside and weld it back up. Then pretend you never saw what you saw.

Is there evidence that anything was ever installed on the roof? Like beacons or antennas or does it have cab lights?