“ In a number of codes of fair competition which have heretofore been approved or submitted for approval pursuant to Title I of the National Industrial Recovery Act, approved June 16, 1933, there have been included provisions designed to limit or prohibit the payment or allowance of rebates, refunds, or unearned discounts, whether in the form of money or in any other form, and the extension to certain purchasers of services or privileges not extended to all purchasers under similar terms and conditions. ”