{ s nameptr "{f.~}{vv~}{ll}{, jj}" format.name$ 't :=

{ s nameptr "{vv~}{ll}{, jj}{, f.}" format.name$ 't :=

RequirePackage{filecontents}
begin{filecontents}{mybib.bib}
@misc{ab:3001,
  author = "Anna Adele Author and Brenda Betsy Buthor",
  title  = "Initial thoughts",
  year   = 3001,
}
@misc{abc:3002,
  author = "Anna Adele Author and Brenda Betsy Buthor and Carla Christina Cuthor",
  title  = "Additional thoughts",
  year   = 3002,
}
end{filecontents}

documentclass{article}
usepackage{natbib}
bibliographystyle{abbrvnat-mod}

begin{document}
nocite{*}
bibliography{mybib}
end{document}