I remember fueling an FM2 at MPV that came from someplace in eastern Connecticut which had a passenger compartment in the lower fuselage with insulated, soundproofed interior, two soft comfortable seats facing each other, sound system, intercom, and blue tinted windows that formed the blue bars in the US insignia on the fuselage side. This whole assembly was shock mounted on rubber donuts for vibration isolation. (It WAS a Wright engine, after all.) I asked him if this effected CG, and he said "No problem, I hold the STC on this and it involves shifting some other stuff forward."The lower fuselage area behind the cockpit on the F4F was huge, easily big enough to take a full grown man through that big hatch on the side, I've even read where they did carry a person in there for short trips.
BTW, those pix of the belly from Wurger and MiFlyer look like the panel/window is held in with pan head Dzus fasteners, not screws. Do you suppose they were interchangeable?
Cheers,
Wes