A savings bank may, in addition to other investments presently or from time to time hereafter authorized by law, invest in corporate interest-bearing securities not otherwise eligible for investment by savings banks;
(1) which are not in default as to either principal or interest when acquired, and
(a) which have a maturity of not less than five years from the date of issue, or,
(b) which are part of an issue, issued in a series or repayable in installments, the whole of which issue has an average maturity of not less than five years from the date of issue.