Read this:
"A common misconception about another feature on the F4U-4, has to do with the windscreen. Most other references on the Corsair have stated that the F4U-4 had a flat windscreen insted for the rounded one used on the F4U-1A, -C and -D. While this flat bulletproof windscreen did appear during the production of the F4U-4, almost half of the F4U-4 's had the earlier rounded windscreen. The 5 protoypes BuNos. 80759 through 80763 and the first 995 production F4U-4's, BuNos. 80764 through 81758, had the rounded windscreen. The following 20 aircraft of the production line, BuNos. 81759 through 81778, had the first flat bulletproof windscreen. These were followed by 50 more F4U-4's 81779 through 81828 which had the rounded windscreen. BuNos. 81829 and the subsequent had the flat windscreen design. Therefore, out of the 2,356 F4U-4's, F4U-4B's and -4P's produced, 1,045, or just over 40 percent, had the original round windscreen."