If I'm not mistaken the Escalade changed engines between those years and also the 6.0 H.O. stands for High Output and has basically 45 more horsepower than a regular 6.0. The Silverado SS, Cadillac Escalade, 2500 HD's and some GMC's have the engine as an option and it is stock on the Escalade and SS as the only engine choice. I may be wrong about the horsepower but am 99.9% sure of the horsepower difference.