Savings banks may invest in the capital stock, capital notes and debentures of any trust company and of any other corporation organized under the laws of this State, provided all the stock of such trust company or corporation is owned by not less than 20 savings banks and subject to the further conditions set forth in sections 63.2, 63.3 and 63.4 of this Part. For the purposes of this Part, such a trust company is referred to as a “savings banks trust company” and such a corporation is referred to as a “service agency”