As you say all of the componentry is completely different, however the frames are essentially the same. The main differences are in the downtube shifter mounts; the '62-'63 has brazed on downtube shifter bosses, while the '76-'78 has a brazed on stop tab to support a clamp-on shifter mount instead. The forks are also different, especially in the crown area. Finally the '76-'78 was also available in a larger 26" frame.