A mutual savings bank may invest its funds in the bonds or obligations of the United States or the Dominion of Canada or those for which the faith of the United States or the Dominion of Canada is pledged to provide for the payment of the interest and principal, including bonds of the District of Columbia: PROVIDED, That in the case of bonds of the Dominion or those for which its faith is pledged the interest and principal is payable in the United States or with exchange to a city in the United States and in lawful money of the United States or its equivalent.