Shimano makes two styles of hubs those with aluminum splines for the cassette and those with steel splines. http://www.flickr.com/photos/63373992@N07/8036521062/
Generally the steel spline hubs have a 2mm longer cassette and the flange is undercut to make room for it. http://www.flickr.com/photos/65657096@N00/7014908657/
This is a weak point and Shimano knows it that is why they do not do this on the tandem hubs. Of course this adds 2mm of dish. On the M529 the did something different they used a steel spline with out undercutting the flange and combined it with an LX level cassette. The hub is advertized as a high torque hub I don't know if the ratchet was made stronger. There is really no excuse for this I snapped the heads off four 1 1/4 inch bolts using a 3/8 inch craftsman ratchet. The hub is rather light for a large flange hub at 440grams. I use the large flange hubs with out the brake because I bent fewer solid axles with these. I not going to buy one of these until I can get one with 36 holes. This is the idea hub to be drilled for 40 or 48 spokes. With 48 spokes I would consider buying a pair and using a front disk brake.