Except as provided in § 223.11, no company holding a certificate of authority shall underwrite any risk on any bond or policy on behalf of any individual, firm, association, or corporation, whether or not the United States is interested as a party thereto, the amount of which is greater than 10 percent of the paid-up capital and surplus of such company, as determined by the Secretary of the Treasury. That figure is hereinafter referred to as the underwriting limitation.