Under this article, the board may select and retain one, or more than one, investment manager who shall invest or reinvest the corpus of the trust fund subject to the requirements of the plan as directed by the board and as set forth in any agreement with the investment manager. As a condition to its appointment, an investment manager shall acknowledge in writing that it is a fiduciary with respect to the plan. An investment manager who is not also a custodian shall not have authority to take custody of any property which is a part of the trust fund. The board may furnish an investment manager with written investment guidelines for investment of the investment manager's account, which guidelines may include directions with respect to diversification of the investments. The board may select a custodian and one or more separate investment managers to invest or reinvest the corpus of the trust, or the custodian and the investment manager may be the same entity. (Code 1979, § 15-53)