The Ruger New Model lockwork was introduced in 1973. According to what I have heard, it was the result of Ruger losing a lawsuit. The new model lockwork, AND refitting every old model that they got their hands on, were part of the settlement.
I can't give you chapter and verse on the details, likely someone here can. I can give you the "urban legend" version, which says, some idiot was loading/unloading his Ruger .44Mag (in the cab of a pickup truck, while moving, he apparently was not the driver) and shot himself in the leg. He sued Ruger.
The jury decided (wrongly in my opinion) that Ruger was at fault. As part of the settlement, the new model lockwork was produced.
This is also why, if you send an old model to Ruger, for any reason, Ruger will convert it to the new model style, whether you want them to, or not. They don't have a choice. Last I knew, Ruger does give you back the original parts, so if YOU want, you can return the gun to original configuration.