The issuance or sale of more than one class of senior securities representing indebtedness by a small business investment company, licensed under the Small Business Investment Act of 1958, shall not be prohibited by section 18(c) so long as such small business investment company does not have outstanding any publicly held indebtedness, and all securities of any such class are (a) privately held by the Small Business Administration, or banks, insurance companies or other institutional investors, (b) not intended to be publicly distributed, and (c) not convertible into, exchangeable for, or accompanied by any option to acquire, any equity security.