Section 56. The provisions of this chapter, to the extent that they relate to the Unemployment Compensation Fund, shall be operative only so long as such Unemployment Compensation Fund continues to exist and so long as the secretary of the treasury of the United States of America continues to maintain for the commonwealth a separate book account of all funds deposited therein by the commonwealth for benefit purposes, together with the commonwealth's proportionate share of the earnings of such Unemployment Compensation Fund, from which no other state is permitted to make withdrawals. If and when such Unemployment Compensation Fund ceases to exist, or such separate book account is no longer maintained, all moneys belonging to the Unemployment Compensation Fund of the commonwealth shall be requisitioned by the state treasurer as ex officio treasurer of the fund. The state treasurer shall thereafter, in accordance with the pertinent provisions of this chapter, hold, invest, transfer, sell, deposit and release such moneys, and any properties, securities, or earnings acquired as an incident thereto, for the administration by the commissioner as a trust fund for the purposes of paying benefits or refunds; provided, that such moneys shall be invested in the following classes of securities:

(a) Bonds or other interest-bearing obligations of the United States of America;

(b) Other direct obligations of the United States;

(c) Securities which are the direct obligations of the commonwealth;

(d) Bonds and other interest-bearing obligations of any legally constituted political subdivision of the commonwealth, if such subdivision has never defaulted in the payment of the principal or interest of any of its bonds or other interest-bearing obligations.