An association may invest its funds in loans upon or purchases of the bonds or obligations of or bonds or obligations guaranteed by the United States of America, including bonds of the District of Columbia, of 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 interest and principal: PROVIDED, That, in the case of bonds of the Dominion of Canada or those for which its faith is pledged, the interest and principal shall be 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.