Exemption shall not be denied any such association because it has capital stock, if the dividend rate of such stock is fixed at not to exceed the legal rate of interest in the State of incorporation or 8 percent per annum, whichever is greater, on the value of the consideration for which the stock was issued, and if substantially all such stock (other than nonvoting preferred stock, the owners of which are not entitled or permitted to participate, directly or indirectly, in the profits of the association, upon dissolution or otherwise, beyond the fixed dividends) is owned by producers who market their products or purchase their supplies and equipment through the association.