Pongo,
I did some checking after you brought this up in the MA earlier tonight. The window was in the early versions of the F4U, but was later deleted. However, before it wqas deleted, it was painted over. Most of the pilots never used it, and some even said they couldn't if they wanted to. They said that it would be covered with trash and dirt that had fallen down from the cockpit. It would cover it up from the inside to the point that it was useless.
-math
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"Any American fighter near Orote Penninsula. I have forty Jap planes surrounded and need a little help."
-Ens. W.B. "Spider" Webb during the Marians Turkey Shoot