In Old Coptic, both ⲥⲏⲧ and ⲥⲏⲑ are attested. If they are not mere orthographical variants, it seems that these forms derive from two different forms of *sutaḫ: The first via *sutə > ⲥⲏⲧ (and written st, ste in Demotic), while the other comes from *sutḫ > ⲥⲏⲑ (maybe indicated by stḫ in Demotic). Vycichl seems to suggest the same. This is also not the usual Bohairic / non-Bohairic divide between t and th.
