Similar thing with the receivers on 1970's Winchester 94. There is a process to make the surface of the receiver more pure iron (passiving?) to refinish those. The castings were "graphitic steel" and they iron plated them from the factory to get them to take blue. Du-Lite has a process to refinish these for the gunsmith.I Love the plum look! Early Ruger loading gates are notorious for turning plum. I was told it was not due to bluing methods but rather the silicon in the steel that was used.