The truth is that the id of a 1/4,3/8/1/2 etc NPT fitting (and any other) can vary between different manufacturers. Even within the same manufacturer. The only way you can tell for sure what the id of the barbed and threaded section are would be to:
measure them
find exact information for that particular piece

The typical ID is another thing....so for everyone that wants the biggest hole in the smallest package, they must jump through the hoops and get that information first hand...YOU CANNOT (or should not) ASSUME that ALL fittings with the same external sizing will have the same internal sizing, it just isn't so.
wj