[Graham Boak]

You are generally quoting major interfaces, on mass production types built on expensive tooling. What about the smaller parts? And did all these parts fit without fettling? If not, they were not "interchangeable".

If everything had been interchangeable (which few if any aircraft ever has been, fully) then there would have been no need for any change of designation. These things are not done just for show, to let the pilot know where the machine was built - why should he care? They are normally done to clarify the logistics, to indicate that differences exist and that the two should not be confused. Generally this is not something that will affect flight qualities - but it has been at times. (eg Blackburn built Swordfish or London Transport Halifaxes, which did not have different variant designations.) When it comes to aircraft variants, it may be because one line used (eg) Sperry parts and the other (eg) Lockheed. Normally, however, this is because a piece taken off one aircraft will not match perfectly the hole of another, and has not been built to the standard for which this can be expected.