Even if they did have one of these on the Tunisia saber, I'm inclined to believe that they still had the neck screwed in to the wrong end of the frag body (the red end in this painted example) in Tunisia. As pointed out earlier, when the neck is screwed in to the wrong end of the frag body, the neck won't go in deep enough to hide all of the threads; and there is photographic evidence that seems to show the Tunisia saber had extra threads showing around the base of the neck. Perhaps the prop makers had a grenade with a frag body like the one in this photo, took that one fully apart, and then accidentally re-assembled it the wrong way around, without realizing. (Having a frag body with two square ends would make that accident more likely to happen.)
Either way, I'm inclined to believe that they built more than one OWK saber, but maybe only one of the ones that they built was accidentally assembled with the neck on the wrong end of the grenade frag body. Perhaps when a prop manager was packing up props to bring to Tunisia, they thought to themselves, "We won't need any close-up shots for the OWK saber outdoors on Tatooine. Which of these multiple sabers should we bring to Tunisia? I know. I'll bring this one here - the one with a more square end on the frag body compared to the other(s) - because it looks worse than the other one(s). It has a few threads sticking out. I don't know why, but who care? I'm not going to try to figure out why it looks different than the other ones. No one will notice. I'll save the better-looking one(s) for the indoor close-up shooting, when we return to England."