Winchester started using that thread on the actions for the wssm cartridges and then began using it on all actions for the wsm cartridges. When the New Haven plant was closed down, all Model 70's were made with the new thread. This eliminated the interrupted thread in the receiver which was causing chamber distortion in the WSM's. I prefer to re-thread all post-64 Model 70's. I stay with 16 tpi but go up to 1 1/16 diameter. GD
