So I have been doing some research into some things in the fandom and in game up that says it's an au but I don't think it's an au I belive it is Canon here me out. 1. In the real world when the jews were being killed by the Nazis women would have the numbers on the inside of the arms and the men would have it out side and were does Erik have his number on the Inside 2. I belive we have never seen Erik shirtless 3. Erik when older asks kids what there real names are you might say that's because there mutants but he would probably say what's your mutant name not what's your real name unless he asking a different question all together.
