From looking at the thread it sounds like there were two types of Stromberg carbs, one with the pressure injection system that was used on most of the engines 1800 in^3 and up, and a float carb version that was used on most of the smaller engines. My guess is that the Gnome-Rhône engines used the float version, and the US versions used the pressure injection system.
As I understand in, the US Navy selected their fighters heavily for acrobatic performance*, so inverted flight was a high priority for them. Both the Spitfire and Merlin engine, on the other hand, evolved out of racing, where inverted flight wasn't a typical mode, so inverted flight systems could easily have been over looked. There is actually a lot of design work that goes into making a plane fully inverted capable, and not all of it is thing that you'd think of right off the bat.
*This was the reason the Brewster Buffalo was initially chosen over the Wildcat, and why the Wildcat had the crazy riveted construction it did. Yes, the unloaded Buffalo, with its one piece wing was more acrobatic than the Wildcat.
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