There shall be a board of trustees consisting of five members. Within ninety days after the effective date of this charter one member shall be appointed by the mayor for a term expiring on the first Monday in January of 1970; one member shall be appointed by the council for a term expiring on the first Monday in January of 1969; one member shall be appointed by the fire force and one member shall be appointed by the police force, each for a term expiring on the first Monday in January of 1968. These four members shall appoint a fifth member for a term expiring on the first Monday in January of 1967. Thereafter members shall be appointed in like manner for two year terms. All members shall hold office until their successors are appointed, anything herein to the contrary notwithstanding. It shall be the duty of the board of trustees to administer the pension fund. The board of trustees shall have complete discretion as to the investment of the pension fund; provided, however, that the board of trustees shall invest only in such securities as are permitted by the laws of the State of Michigan for investment of trust funds of a city retirement system (or policemen and firemen retirement system, if more restrictive).