In NLRB v. Pittsburgh Steamship Co., 340 U.S. 498, 71 S.Ct. 453, 95 L.Ed. 479 (1951), the Court held that an employer's letters to employees on the eve of a union election were not substantial evidence of an unfair labor practice, but the Court "express[ed] no opinion on the possible effect of ยง 8(c)" on the inferences permissibly drawn from those letters.