It's not armor at all.
It was built there as support for the canopy glass to prevent that glass from cracking as the canopy slid back.
Kazaa, you may be looking at later, post-war F4U's. In 1945, F4U-4's and even F4U-1D's were getting rid of that canopy cap, and by post-war times, F4U's no longer had them at all. No Korean War F4U's you find will have them. I have a feeling you may be looking at Korean War Corsairs as the majority of F4U's had the cap in WWII.