There is a TKO 500, which is the newest version of what used to be called the 3550. It has a new alloy gearset, steel forks, and some other upgrades from the 3550, and it is rated at 500 ft-lbs. Still has the same 10-spline input shaft that the 3550 had, though. The TKO 600 is what used to be called the TKO, and differs from the TKO 500 in that it has the upgraded input shaft, just as the TKO differed from the 3550. The TKO 600 is rated at 600 ft-lbs. D&D performance shows all of a $30 difference in price between the two, so I think I could tell you which one I'd get.