A nonprofit cooperative association may assent on behalf of any of its members only if it is authorized so to do by an instrument in writing which is signed by the member.

Such authority may be revoked as to him by any such member by an instrument in writing which is filed with the director and with such association. The revocation shall become effective three days after its receipt by the director. A copy of any written authorization, or any revocation of such authority, of a producer to the nonprofit cooperative association of which he is a member shall be forwarded to the director by the association.