Grumman Martlet Mk. IV question

Due to the different weight of the R-1820, to preserve the CoG the section of the fuselage behind the cowling was longer on the IV as well.

http://www.vectorsite.net/avwcat.html#m3

The USN obtained 1,169 F4F-4s. The FAA obtained 220 “Martlet IVs” that were based on the F4F-4 but had substantial differences, most particularly the fit of a Wright R-1820-40B Cyclone in a distinctly more rounded and compact cowling, with a single double-wide flap on each side of the rear and no lip intake. These machines were also referred to as “F4F-4Bs” for contractual purposes.

The USN F4Fs (Grumman) were all powered by a Pratt & Whitney Twin Wasp, as were the FM-1s (General Motors).
The FM-2s from GM had the Wright Cyclone.

Martlet I [ex-French order]: Wright R-1820 Cyclone; fixed wings; 4 guns; 81 delivered to UK (+10 lost en-route); Note that these aircraft were in RN service (deliveries from July 1940, and shooting down a Ju-88 over Scapa Flow 25 December 1940) before the USN formally accepted their first F4F-3 Wildcat (January 1941)

Martlet II [RN order]: P&W R1830 Twin Wasp; folding wings; 6 guns; 90 delivered to UK from August 1941

Martlet III(A) [RN order]: P&W R1830 Twin Wasp, fixed wings (refitted with folding); 4 guns; 10 delivered to UK from April 1941

F4F-3/F4F-3A/Martlet III(B) [ex-Greek order]: P&W R1830 Twin Wasp, fixed wings; 6 guns; 30 delivered to UK (taken over in Gibraltar April 1941)

Martlet IV [RN order]: Wright R-1820 Cyclone, folding wings; 6 guns; 220 delivered to UK

F4F-4: P&W R1830 Twin Wasp; folding wings; 6 guns

FM-1/Martlet V [RN order]: P&W R1830 Twin Wasp; folding wings; 4 guns; 312 delivered to UK

FM-2/Wildcat VI [RN order]:Wright R-1820 Cyclone, folding wings; 4 guns; 370 delivered to UK (RN accepted the US name for these)