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I guess that ""ɑɾɹ̩"" for ""otter"" is a misprint in the source and should read ""ɑ̆ɾɹ̩"". Evidence for this is that where Silverman describes the normal outcome of tapping with distinction marked by length contrast on the preceding vowel, we read: ""*ˈVtV > ˈV̆ɾV"" (Silverman 2004, p. 87). Also, this is consistent with how the length contrast on the preceding vowel is marked in other surroundings (e.g. in the cited table in ""knot"").

With regard to ""nod"", I think there is nothing wrong. It is pretty much the point of Silverman's article that in this particular position, de-voicing is common. Of course, additional sources would be nice.   🙈🙉🙊 "