Wow… that is some complicated nonsense. Vader is Luke’s father but not Anakin… that makes no sense at all. The name Anakin hadn’t come up yet when Vader, in James Earl Jones’s voice (or in the novel if you read that first), said he was Luke’s father. We have no idea when Lucas decided that was going to be the story, but script hints at it with the consistent lack of any line at that location (referring instead to inserted dialog). We only know that sometime between the dated first draft in Feb 78 and the completion of filming in Sept 79 that Vader was Luke’s father. It could have been earlier (and not included in what he gave Leigh Brackett on the story), but we really don’t know. It was almost certainly in the script that the novelization writer worked off of, but that was about the same time.

Lucas claims he had that in mind from earlier, but with his track record at reconning the past, he can’t be completely trusted. But, others can. Hamill, Kershner, and Kasdan had no reason to lie and according to them it was intended that way from Hamill’s performance on set and had been that way in the full screenplay (not the shooting copies given to the crew) for quite some time. The earlier drafts of the screenplay bear this out by having a reference to inserted dialog. According to the book Star Wars: The Annotated Scripts (by Laurent Bouzereau), it was the second draft that had the first version of the iconic “I am your father” scene. That was the draft Lucas did himself before Kasdan came on board. This doesn’t quite back Lucas up, but it does place it very early in the story development.