Under § 8 (b) (4) (ii) (B) of the National Labor Relations Act, as amended,[1] it is an unfair labor practice for a union "to threaten, coerce, or restrain any person," with the object of "forcing or requiring any person to cease using, selling, handling, transporting, or otherwise dealing in the products of any other producer. . . or to cease doing business with any other person . . . ." A proviso excepts, however, "publicity, other than picketing, for the purpose of truthfully advising the public . . . that a product or products are produced by an employer with whom the labor organization has a primary dispute and are distributed by another employer, as long as such publicity does not have an effect of inducing any individual employed by any person other than the primary employer in the course of his employment to refuse to pick up, deliver, or transport any goods, or not to perform any services, at the establishment of the employer engaged in such distribution." (Italics supplied.) The question in this case is whether the respondent unions violated this section when they limited their secondary picketing of retail stores to an appeal to the customers of the stores not to buy the products of certain firms against which one of the respondents was on strike.