My understanding of Kutter's comments was that there was a consistent difference in both feel and effect upon his graver tips on Model 21 actions versus the trigger plates. Yet he said they are supposed to be the same grade of steel. If that is true, then the only explanation for tougher cutting in the action would be different heat treatment. I considered the possibility of inclusions such as scale, which is notorious for eating up cutting tool edges and breaking taps. But he seemed to suggest that he has experienced this difference on more than one gun. And it sounded like he has had similar observations about other Winchester actions and receivers,
Your comment about possible minor differences in bluing shades got my attention, combined with Kutter's thoughts that perhaps the receiver ring of Model 70's has a different localized heat treatment than the areas to the rear. I have a commercial Mauser model 66 S that is pristine except for the fact that the hot blued receiver ring has acquired that purple tone associated with higher nickel alloy steels. Yet the rest of the action remains a deep blue-black. I suppose that I could check to see if the heat treatment of my receiver ring is different than the rear of the action by accepting a couple little dimples from a Rockwell hardness test. But it would probably be a better investment to simply have it hot tank reblued.
