There isn't one enlarged original-team lineup that covers all these stories. The changes are part of the story itself.

There isn't one enlarged original-team lineup that covers all these stories. The changes are part of the story itself. X-Men #27–29, 1966–67: Mimic briefly joins the original five. His departure comes well before the arrival of Lorna and Alex. The expanded lineup lasts only a short time, then the original five carry on without him. #39, 1967: the original five receive individual costumes after Factor Three. They are still the same people, but the visual presentation finally gives each of them room to look like an individual. #42, 1968: the team believes Xavier has died. #65, in 1970, later explains that Changeling was the man killed while impersonating him. The revelation belongs at the end of that publication sequence; it isn't something the grieving students knew in advance. #49–52, 1968–69: Lorna Dane enters the cast through the Mesmero story. Her introduction and the activation of her magnetic powers precede her participation as a teammate. By #60, she is training with the X-Men. #54 onward, 1969: Alex Summers enters as Scott's younger brother. The Living Pharaoh and Sentinel stories develop his powers and Havok identity. #65 gives a clear snapshot of him working with the team in costume, alongside the original five, Lorna and Xavier. These changes leave the original cast with a wider circle and a less straightforward history. An ally can arrive through a rescue, a former opponent can contribute, and someone can serve the school without the others even knowing his face. That is the version of the late original team worth keeping distinct from the founding class: familiar people, with the certainty of that first classroom gradually giving way to a much messier shared life.