In the absence of any express provisions to the contrary, whenever any general statute, special act, regulation, trust indenture, will or other instrument governing the investment powers of trustees directs, requires, authorizes or permits investment in United States government obligations, a trustee may invest in and hold such obligations, either directly or in the form of interests in an investment company or investment trust registered under the Investment Company Act of 1940, (15 USC, § 80a-1 et seq.) as from time to time amended, the portfolio of which is limited to United States government obligations and to repurchase agreements fully collateralized by any such obligations, and which is rated within the top two rating categories of any nationally recognized rating service.