I hesitate to say that S&W's metallurgical improvement timeline is a secret. When I began buying their revolvers in 1970, I remember that these discussions were underway then. The consensus was that then they differed in caliber and frame size but not metallurgy. I no longer can provide citations or sources but had many conversations with the product service manager during the 70s. The Smith forum should be able to shed light. I can do some digging and find a phone number for the man who had the last warranty repair shop in Texas. He's still in the general gunsmith business. I would pm the info so one of you could call him.