Brief for National Labor Relations Board, United States Court of Appeals for District of Columbia circuit, no. 10271, Denver Building and Construction Trades Council; International Brotherhood of Electrical Workers, A. F. L., Local 68; and United Association of Journeymen & Apprentices of Plumbing and Pipe Fitting Industry of United States and Canada, A. F. L., Local 3 v. National Labor Relations Board, on petition for review and on request for enforcement of order of National Labor Relations Board.