A controlling authority may, in its discretion, grant or refuse any permission mentioned in subsection (1) of section two for which any person has applied, and if it grants such permission it may (subject to the applicable provisions of any regulation made under paragraph (b) of subsection (1) of section fourteen), prescribe the specifications to which the advertisement whereto the permission relates, shall conform, the period during which the permission shall be of effect, the manner, place and circumstances in which and the conditions on which the advertisement may be displayed, and may at any time alter or revoke any such permission.