The F4F-4 was the first version of the Wildcat to feature a Grumman innovation, the Sto-Wing. The Sto-Wing used a novel approach using a compound angle folding-wing that was unique to Grumman. Leroy Grumman developed the idea by experimenting with a rubber eraser and a paperclip. He inserted the paperclip into the eraser as if it were the wing of an airplane until he found the correct angle where the wing would fold flat alongside the fuselage. However it took a lot more engineering to fully develop his idea to make the wing strong enough. It was a successful design that was later used on the F6F Hellcat and TBF Avenger. Total production of the Wildcat, excluding prototypes, was 7,898 aircraft, with 5,927 produced by General Motors. General Motors built Wildcats carried the designation FM-1/2 and operated primarily from escort carriers.