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The text of our Targum Onkelos is from Chumash Toras Elokim (Vilna, 1874) which includes R' Wolf Heidenheim’s text of the Targum; that text is based on manuscripts and is considered מדוייק.
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If someone has this version please verify. I'm not marking this answer as correct because it doesn't explain the underlying reasons why Nachshon is different than the others.