Best thing to say is that the AC series is mainly built up with loosely based sequels the only dramatic change in story being AC:AA to AC3 which seemed to be more of an attempt to reboot the series rather than continue the story that started in AC 97. Otherwise it is safe to say that humanity in the AC universe is constantly going to war and some genius digs up ancient technology and uses it to make a super computer to force humanity into a position so It can't spread it's virus across the universe blah blah. However, there is no real reference to "ancient" tech in AC4 and ACFA, only outdated weapons and machines that all have a clear past. The only pseudo alien like weapons being the newest and most recently built Kojima powered Arms Forts, Particle Cannons, and the NEXTs themselves. From this I believe it is safe to say that there was no true Normal AC as the way we would imagine them. None of the Normal squads show any sign of customization, all of them clearly mass produced, even the ones that are supposedly elite squads. I believe in the 4th gen series that the NEXTs were in fact the first ACs to utilize interchangeable parts tech. To that theory I would push forward that 4th Gen NEXTs could very well be considered the Grand Fathers of all true Armored Core.