It's hard to nail down exactly when in the Kamino battle that he gets the dent, and part of that is the fault of the CG animators. I grabbed some quick screen shots...
First, the money shot of Jango that was in the first trailer. Right at the beginning of the battle he turns to see Obi-Wan coming. **Human/no dent**
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Then, right after that is the CG model of Jango about to take flight. Here, he does have the dent **CG model/dent**
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Next is when he fires the rocket from his pack, and the dent is still there...(notice the range finder aimed at his back too) **CG model/dent**
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Yet, after he crash lands and loses his pistol he does not have the dent. (blurry pic, hard to get a clean shot of this as it is literally only a second or two long) **human/no dent**
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But after he gets kicked by Obi-Wan and headbutts Obi-Wan the dent is present. It's also after he crashes to the ground losing the jetpack, but I can't get a clear shot of his helmet in between those two scenes. **human/dent**
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So, it's in there that the costumers switched to the dented one.
