I don't agree with Ruger on that, my I understand it.
The reason the New Model exist is Ruger's protection against idiots and lawyers that will sue them if another idiot hurts someone while handling and Old Model. To sell a part (even a spring) that puts an Old Model in working order, is the same as them selling an Old Model.
Can't you just hear the attorney in court blaming Ruger?
I love Ruger until the changed to the New Model and started labeling the metal parts with those stupid warnings.
If I had the money, I'd get a Colt, but I don't, so I just fantsize about the 4 clicks and the half-cock loading and the only true perfect mating of a gun to a hand.