I think Manuel is right...pull a top-and-bottom-vented tank apart and you'll see each vent run to nearly the opposite wall. Vents the same upright or inverted. You could plug the filler tube on your single-vented tank, after filling, and it wouldn't leak inverted but probably would run leaner, having to pull air through the fuel load then. Cox made several different models of engines with the Black Widow-style oversize tank, some double-vented, some not...the Golden Bee comes to mind, and I know there are some others--the Texaco, maybe? Others will know.
--Ray