Shares may be issued to participate fully or to a limited extent in the net earnings of the association if the articles of incorporation so provide, and such participation shall be specified in the body of the certificate. All shares participating fully in net earnings shall be entitled to an equal rate of dividend, if earned, as fixed by the board of directors. Shares participating to a limited extent shall be entitled to such rate of dividend, if earned, as fixed by the board of directors for the semiannual periods ending June thirtieth and December thirty-first of each year; but, if the rate is decreased, the shareholders thus affected shall be given written notice of the new rate, within thirty days after date of declaration, by mailing such notice to their last known addresses.