I’m interested in comparing ratings of 15-year-old irons to current models. Is this valid? Some older Callaway irons have very high MPF ratings, it seems like something may have changed in the MPF methodology or measurements, to shift MPF ratings.
The method of measuring an irons MPF has not changed. That is the great thing about it. You can truly compare the mass and dimensional characteristics of irons from one generation to the next. These mass and dimensional characteristics and how they affect playability never change. As long as you understand that you are comparing the specific mass and dimensional characteristic measurements (vertical cg, horizontal cg, rearward cg, MOI) that Ralph outlined, you can truly make comparisons.
Are you saying the dimensional characteristics have not changed, but the MPF rating formula has changed?