A mutual savings bank may invest its funds in capital stock, notes, bonds, debentures, or other such obligations of any corporation which is or hereafter may be created by the United States as a governmental agency or instrumentality: PROVIDED, That the total amount a mutual savings bank may invest pursuant to this section shall not exceed fifteen percent of the funds of such savings bank: PROVIDED FURTHER, That the amounts heretofore or hereafter invested by a mutual savings bank pursuant to any law of this state other than this section, even if such investment might also be authorized under this section, shall not be limited by the provisions of this section and amounts so invested pursuant to any such other law of this state shall not be included in computing the maximum amount which may be invested pursuant to this section.