No such insurer shall expose itself to loss from all causes, on any 1 risk, in a total principal sum exceeding 10% of its contingency reserve deposit as defined in section 6434: Provided, That insurers may establish, make and maintain a larger contingency reserve deposit than is required by said section 6434, and thereafter the limits of exposure as herein defined shall be increased by 10% of any such excess deposit: Provided further, That insurers transacting business under this chapter may issue policies under which the amount insured exceeds the foregoing limitations, if the said insurers shall, at the time of issuance of said policy or policies, have in force, and shall maintain in force throughout the continuance of such policies, a good and enforceable contract or contracts of reinsurance ceding of said individual risks not less than the excess over the permissible retention as herein provided, which reinsurance shall be ceded only to insurers authorized under the provisions of this code to transact business of a similar class in this state, and to accept reinsurance: Provided, however, That no reinsurance shall be ceded to or accepted by any insurer operating under the cooperative or assessment plan: Provided further, That this section shall not reduce the limits of exposure to loss for insurers legally operating in this state under the cooperative or assessment plan on January 1, 1947.