These are the same model piston, KB 118 for use in a 350 rebuild. I'm assuming there must have been a change in design at some point. All the measurements are the same except for the height of material on the top of the piston. The straight edge relief has a thicker center section dividing the rings. The dual valve reliefs design is equally spaced. anyone know if one design is newer or older? Perhaps one design is stronger than the other? Posted using the Full Custom H.A.M.B. App!