I don't think Eraqus' symbol represents all Keyblade mastery, because Eraqus and Aqua are the only masters so far to possess that symbol, and Terra, Aqua, and Ven all had that symbol while they were all apprentices... Plus, Ven didn't have that symbol while he was Master Xehanort's apprentice but he obtained that symbol when he became Eraqus' apprentice. Also, when Eraqus' symbol is turned upside down and slightly modified, it resembles the Nobody Symbol. Xemnas probably designed the Nobody symbol, and he is after all, the vessel of Terra. Plus, one of Xemnas' last forms possesses a structure that looks like a Nobody Symbol turned upside down, which again slightly resembles Eraqus' symbol. A slightly modified version of Roxas' symbol can arguably be seen on the Way to Dawn. Maybe Roxas' symbol was just meant to hint that he was a special nobody... before KH2 came out of course. Like... maybe his symbol was supposed to show a connection to the Organization... a hint for the audience before the release of KH2.