Milder lead bullet loads of any fast to medium burning rate powders will be satisfactory in any K-frame, old or new. I don't see that the lead hardness would have much of anything to do with their safety. Sharpe's reloading manual of 1937 was recommending .38 Special loads which are about the same as those recommended today. And there were far more old S&W revolvers around in 1937 than there are today. For any recreational shooting with a .38 Special I use only lead bullets.