No. 91-1036. Argued October 25, 1991. Decided December 27, 1991. Donald B. Greenspon, Detroit, Mich., for petitioner. William A. Baudler, Atty. N.L.R.B., with whom Jerry M. Hunter, Gen. Counsel, and Aileen A. Armstrong, Deputy Associate Gen. Counsel, Washington, D.C., were on the brief, for respondent. Beverly Oyama, Atty. N.L.R.B., Washington, D.C., also entered an appearance for respondent. John S. Schauer, with whom Camille A. Olson, Chicago, Ill., was on the brief, for intervenor. Petition for