Here, I've got to defer to those who actually KNOW the answer. The glittering generalities were easy but there's a lot of nitpicky details since the IRT had a tendency of rebuilding and reusing things, and modifying others. Big Ed, Uncle Joe Frank and several others have actually studied the extreme minutiae of all this. Hopefully it won't be a long wait until one of my buddies comes along and gives you a proper dissertation. :)