The impression I’ve always had is that the salt shaker prop was designed as a replacement for the saber lost in AOTC. Then, during the course of ROTS, Obi-Wan would lose the salt shaker and build or acquire the familiar thin-neck ROTS/ANH-style saber.
There’s a moment in this ROTS webdoc—
—beginning at 5:27 where Lucas is being shown the hero saber props built for the film. He indicates the Anakin and Vader sabers, then (in a notable camera cut) says, “So all I have to do is get rid of this one, somehow”.
HOWEVER, the salt shaker and the Obi ROTS sabers are also sitting on the table, and Lucas indicates the Obi ROTS hero and says, “So this is the one that Alec Guinness had, basically”. I strongly believe that Lucas was actually referring to the salt shaker when talking about getting rid of “this one”, because Obi-Wan obviously needed to have his ANH-style saber by the end of the film. But, due to clever editing, it appears that Lucas is talking about getting rid of Anakin’s saber during his transition to Vader, which then leads to the clip from ANH with Kenobi giving the saber to Luke.
I suspect that the salt shaker was only used for stunt practice and early costume and camera tests (such as those shots of McGregor and Christensen in Padme’s apartment set, despite no scripted scene featuring both of them at that location in the film, akin to Christensen being photographed on the Tantive IV set). I think that Lucas must have decided to ditch the prop entirely before filming began, and to simply have Obi-Wan using his ANH-style saber from the very start of the film.