Wow and here I thought that plate was there to keep the sun off my noggin all these years.
By the way is this meaningful?
The F4U-1A incorporated a semi-bubble canopy (replacing the earlier 'birdcage' type), with only 2 reinforcing bars in the upper surface. A taller tail wheel strut was also incorporated (lengthened by 6.48 inches). All subsequent F4U-1A's, as well as Goodyear FG-1D's (# 1001 onwards), and Brewster F3A-1's (# 650 onwards) had the R-2800-8W engine. The built-in bounce of the oleos was eliminated once and for all in 1944.
The F4U-1B was an F4U-1A featuring a fuselage centerline hard point and clipped wings (delivered to the British Fleet Air Arm).
Incorporated into the first F4U-1C was the installation of a Jack & Heinz electric starter in place of the starter cartridge. The -1C was cannon-equipped, with 20mm cannons in place of the 6 machine guns. It was first delivered in June 1944. Goodyear and Brewster made no equivalent model of the -1C. On the -1C and -1D, the 63 gallon leading edge wing tanks were deleted. Very few of the -1C were actually produced.
The F4U-1D appeared off the assembly line in April 1944. The Goodyear equivalent, the FG-1D, commenced rolling off the Akron, OH. assembly line in September 1944. The Brewster F3A-1D did not go into production, as Brewster went out of business in July 1944. The F4U-1D (Bu # 57583 on, being the 803rd F4U-1D, and the FG-1D, had a completely clear bubble canopy which became known as the clear-vision canopy, unlike its predecessor whose canopy was 'bubble' but not completely clear, still with the two reinforcing plates, one each side. During production of the -1D, a cut-out step was added to the right inboard flap to facilitate entry to the cockpit.
http://webzoom.freewebs.com/cafcorsair/Corsair%20The%20History%20Unfolds%20Page%202.html