Is that the difference? I'm wondering because the info I have on the M529 and M629 hubs is that they are designed to withstand the extra torque the new 12 x 36 mtb cassettes generate, but I don't have any info on exactly what the difference in their design is. These new cassettes with the extra large cogs are fairly common on 29er mountain bikes because of the difference in gear inches the larger diameter wheels make.

So they've designed these hubs with extra points of engagement and that will presumably help with the extra torque loads?