Seu Oth, aqui tem uma discussão sobre isso.
Dela, pincei esta parte:
"The only clear cut external distinctions between the F6F-3 and the F6F-5 were the canopy and the aileron trim tab arrangment. The -5 had the bullet resistant glass integrated into the windscreen giving it a flat front panel. The -3 had the bullet resistant glass mounted internally with the canopy having a round front panel. The -3 had a servo tab on the left aileron and a fixed tab on the right. The -5 had a servo tab and fixed tab on each aileron. Many of the other features overlapped the -3 and -5. For example, provision for aerial rockets started with the 3,451st F6F-3, BuNo. 42185. The lower cowl flaps were deleted on F6F-3 BuNo. 39999 (1,265th F6F-3). The bulged engine cowling fairing was deleted shortly thereafter. The early F6F-5's had the windows behind the cockpit. The later -3's and all -5's had the access for the ADI tank on the right side behind the cockpit, etc.
If there were a Peterson's Field Guide to USN WWII Aircraft, the color drawing would show the -3 and -5 in flight, the perspective from slightly above with the bars highlighting the principal differences pointing to the canopy and the aileron trim tabs. In general, an overal Gloss Sea Blue F6F would almost certainly be a -5. A Hellcat in two or tri color schemes would almost certainly be a -3. For a quick positive ID, look at the canopy if visible."