No board of trade whose members are engaged in the business of buying or selling agricultural products or receiving the same for sale on consignment in interstate commerce shall exclude from membership in, and all privileges on, such board of trade, any duly authorized representative of any lawfully formed and conducted cooperative association, corporate or otherwise, composed substantially of producers of agricultural products, or any such representative of any organization acting for a group of such associations, if such association or organization has adequate financial responsibility and complies or agrees to comply with such terms and conditions as are or may be imposed lawfully on other members of such board: Provided, That no rule of a board of trade shall forbid or be construed to forbid the return on a patronage basis by such cooperative association or organization to its bona fide members of moneys collected in excess of the expense of conducting the business of such association.