An open qualification will not normally be approved for the issuance of debt securities unless they are issued under a trust indenture complying with the provisions of the Trust Indenture Act of 1939, whether or not the indenture is required to be qualified under that Act. A limited offering qualification may be approved for the issuance of debt securities which are not accompanied by a trust indenture or which are issued under a trust indenture not complying with all of the provisions of the Trust Indenture Act of 1939, if the trust indenture or other instruments provide adequate protection to the holders of the debt securities under the circumstances of the particular case. The indenture should provide that the trustee or a co-trustee must be a bank or trust company, or if a bank or trust company is not a trustee or co-trustee that any person who is to act in that capacity must have the written approval of the Commissioner.