Any grant of oyster, clam or mussel ground, made by the duly constituted state or town authorities, which is not in territory that has been designated as natural oyster ground by the General Assembly or by the courts, and which has remained in the possession of the owner or owners for five years, shall be valid; and any proceeding to invalidate such grant shall be brought within five years after such grant was made, otherwise the title to such grant shall become complete in such owner or owners.