I’ll be honest, the whole idea of Luke using (or at least appearing to be using) Anakin’s lightsaber as a symbolic turnaround from him throwing it over his shoulder at the beginning of the movie never even occurred to me until now. Even though it’s all an illusion, I was always puzzled by why he wasn’t using his green saber from RotJ. Other than the flashback, do we ever see it at all, even if it’s just lying around somewhere in his hut on Ahch-to?

Did he still have it on his belt in that final shot of the first flashback where he’s kneeling next to Artoo watching the training temple burn?

Since Kylo Ren saw Anakin’s saber get broken in half, wouldn’t that have been a dead giveaway? It makes a little more sense that he doesn’t immediately figure out what’s going on if we assume that Luke’s green saber got destroyed on the unnamed (unless it’s in one of those DK books or something) training temple planet and thus a rebuilt Anakin saber makes a little more sense. But still, that’s a pretty short turnaround time.