Internal A-Pillar Rot, What is This Panel?

[61CustomCamper]

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.