  As a continuation of the authors and Wakatsuki's previous paper [5], we study
relations among Dirichlet series whose coefficients are class numbers of binary
cubic forms. We show that for any integral models of the space of binary cubic
forms, the associated Dirichlet series satisfies a simple explicit relation to
that of the dual other than the usual functional equation. As an application,
we write the functional equations of these Dirichlet series in self dual forms.
