Toshi- The last letter is the Navy designator for the manufactuer. "F" was for Grumman, "M" was for General Motors, who built the same plane as Grumman, under license, as Grumman didn't have the ability at the time to build the numbers required, along with the Hellcat and Wildcat.
As an aside, Goodyear built the Wildcat, under license, as the FM-2, later ones had a taller tail.