There were no announced optical changes from Version 2 to version 3. However, Leica/Leitz has been known to make unannounced changes. For instance, There was a change in the rear element group of the Version 1. They changed from two thin singlets to one cemented doublet. (I think that happened going from version 1 to Version 2, can't recall for sure.) And there have been coating changes.
So there could have been some unacknowledged tweaks between versions 2 and 3, as well.
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