A bank or trust company authorized to act in a fiduciary capacity and acting in such capacity, other than merely as agent, may invest funds held by it for investment as fiduciary in fractional undivided interests in a common fund composed exclusively of property permitted for investment by the terms of section 1002 of this title and of cash, if such common fund shall have been created and is managed exclusively by such bank or trust company as trustee under a written plan, an original copy of which, executed by such bank or trust company, has been filed and is recorded in the office of the clerk of the district court. Under such plan it shall not be permitted that any such fractional interests shall at any time be owned by any other than such bank or trust company as fiduciary under will, under agreement, under court order, or as a guardian, or as executor or administrator. At least once each three months, as of a predetermined date, the fair value of the assets of the common fund shall be ascertained, and a fractional interest in such common fund may only be acquired or redeemed as of such predetermined date by payment, in the case of acquisition, of an amount in cash or United States Savings Bonds Series G at par therefor, or in the case of redemption, of an amount in cash or in kind, or partly in cash and partly in kind, equal to its proportionate part of the fair value of the common fund. A reasonable period, not to exceed seven days, following each such predetermined date may be used to make the computations necessary to determine the value of the common fund and of the participations therein. At the time of the making of an investment in such common fund there shall not be held in the fund any asset which, because of the nature of such asset, the bank or trust company might not then properly purchase as an investment for the fiduciary account for which the investment in the common fund is made. The bank or trust company shall not charge a fee or commission to the common fund for its management or receive any fees or commissions from any fiduciary estate which may be invested in a common fund other than those it would be entitled to receive if such estate were otherwise invested.