The Monogram kit is an F-4J. The SB conversion provides the outboard slats, inboard slat mechanisms, corrected rear cockpit pieces, ECM antennas under the wings/intakes and the wingtips, belly reinforcement strap, trunnion plates on top of the wings, flame holders for the exhausts, and corrected wheels. They are small things that to the casual observer aren't very noticeable, but they change the J into an S.