Earlier F4F-3 fighters did not have folding wings and were armed with four .50 cal MGs. Wing folding arrived with the F4F-4, along with six MGs and many complaints about the added weight and reduced duration of firing more than offsetting the increase in hitting power.
The FM-1 was an F4F-4 built by General Motors. Later, they introduced the FM-2. It was fitted with a 9 cylinder R-1820 engine (the F4F-3, F4F-4 and FM-1 were powered by a 14 cylinder R-1830 engine). Armament was reduced to four guns. Overall weight went down, and power was up. General handling and climb was similar to the F4F-3, but the wingfold remained. A taller rudder was added to enhance low speed handling.
My regards,
Widewing