According to "F4U CORSAIR", "in detail & scale", "PART 1 XF4U THROUGH F2G" (Aviation Publication/ Bert Kinzey):
"F4U-1s had a window in the lower fuselage, but it was rectangular in shape rather than being oval shaped like the one on the prototype. This window was on some early F4U-1As as well, but it was deleted during production of that variant. The windows were painted over on some aircraft, and they were not considered useful (Vought)".
From my other references, it is difficult to determine, as most photos are not of underside AND it is usually too dark when they are. There are some scale drawings which show the windows on F4U-1Ds, but I have found other errors in those drawings and therefor cannot trust them to be accurate. From eveything I have seen, I can say that they were definately not on the later models, anything later than a -2. I think some of the full scales flying these days had that window installed, just as they have installed windows aft of the canopy (usually dark blue tint) so a passenger sitting behind the pilot can have some visibility.
A detail that I think would not only stand out, but also be guaranteed to be make the aircartf more scale, is the addition of the engine compartment exhaust 'scoop' (reversed scoop) under the fuselage, which was in the same area as the window, only farther forward. I have seen this on some of the newer ARFs.