Man its my day for typos it seems.
Sorry I meant F4F not F6F (Hellcat). The FM-2 was a later variant of the F4F which continued to be used as it was smaller than the Hellcat and could take off from the shorter flight decks of the "Jeep" Escort CVs. The FM-1 & 2 were sometimes called the "Wilder Wildcats" as they had a more powerful engine (Wright R-1820) yet weighed less than the original wildcat.
I finally found the critical Spec differences between the original F4F-4 and the FM-2 in my library:
Max Speed: 318 (F4F) vs. 332 (FM-2)
Armament: 6X.50cals, 200lbs of Bombs (F4F) vs. 4X.50cals, 6XHVARs or 500lbs Bombs (FM-2)
Range and Climb rates also improved. While the FM-2 had one less gun

per wing, it did have a higher ammo load per gun

.
BTW - Mitsu, where did you get these pictures you mention.
- Seagoon
(Hoping he didn't make yet more errors)