Pipz I dug out my hellcat Aces of WWII and My squadron Signal F6F Hellcat in Action. I cannot find anything on late model F6F-3s having the rear window removed.
I found a site claiming the the later models F6F-5s had the rear window removed.
[3] F6F-5 HELLCAT IN SERVICE / XF6F-6 / POSTWAR SERVICE* The second (and last) major production variant of the Hellcat was the "F6F-5", which performed its first flight on 4 April 1944 and entered production at the end of the month. The F6F-5 was an incremental improvement on the F6F-3, standardizing improvements introduced during F6F-3 production and adding a few new ones. The two variants were difficult to tell apart. The F6F-5 featured:
The R-2800-10W engine, as fitted to late-production F6F-3s.
Minor airframe changes, including a redesigned engine cowling with improved streamlining, a stronger tail assembly, and new ailerons. The Hellcat's ailerons control was regarded as heavy and tiring, partly because the machine was so stable laterally, and it was one problem with the machine that Grumman never quite managed to fix. The F6F-5 also featured a simplified canopy with the flat windshield, the original curved windshield haven proven to have no real aerodynamic advantage. Late production F6F-5s would delete the canopy rear window panels.
A stores pylon under the fuselage and one under the wing, for a total of three, each able to carry a 450 kilogram (1,000 pound) bomb. The Hellcat would also be evaluated as a torpedo-bomber, but this scheme would not go into service, though the big "Tiny Tim" unguided rocket was used in combat by Hellcats.
Three stub pylons under each wing, for a total of six, each able to carry a single 12.7 centimeter (5 inch) "high velocity air rocket (HVAR)".
The weapons pylons were also fitted to some late-production F6F-3s. Some late-production F6F-5s had gun armament of four 12.7 millimeter Browning machine guns and two 20 millimeter Hispano Mark II cannon, with the long-barreled cannon mounted in the inboard position and supplied with 200 rounds of ammunition each. I guess it depends where you get your info.

I do know you can't base versions on paint schemes.
Below is an FG-1D Corsair. A Goodyear built F4U-1D basically. It's part of the collection at the Cavanaugh Flight Museum in Addison TX.
I took this photo last October. As you can see it has the tri color paint scheme. This aircraft was built in 1945 so it should be all blue.
Notice anything else? Check out the guns...

I posted picture back in October asking if and Goodyear made Corsairs had 20MM Cannon. The answer is no but Guppy35 explained to me it's probably a case of C model wings being installed on this FG-1D for looks more than anything.. As well as the paint scheme. By the way the bird is painted in VF-17 colors. In 1945 when this bird was built VF-17 was flying the F6F. So you can't go by paint jobs or even armament it seems when identifying restored warbirds.
Here's a link with a writeup about the Corsair above.
http://www.cavanaughflightmuseum.com/graphics/Corsair2.jpgTwincam what is the name of the Grumman Test Pilot you mentioned? Thats a guy I'd love to spend a few hours with.
