Unless expressly provided otherwise in any instrument specified in subsection (a) of section 1003 of this title, a bank or trust company acting as a fiduciary and authorized so to act may retain in a trust estate shares of its own capital stock acquired in any manner referred to in section 1004 of this title, as effectively as though the instrument creating or defining the fiduciary's duties and powers expressly so provided, subject to the limitations of the standards set forth in section 1002 of this title.