All Smith & Wesson hand ejector revolvers, and some top-break models, have a provision within the mechanism to rebound that hammer after a cartridge is fired. That is to say that the hammer (and firing pin) is rotated backward far enough so that the firing pin is drawn into the breech face far enough so that the firing pin doesn’t rest on the cartridge case head. This is accomplished by a part called the rebound slide that serves several purposes. First it cams the hammer backward and then blocks if from further movement, and second, it pushes the trigger forward. I’ll call this “safety #1.
As a safety device it was flawed, in that the hammer was blocked at the very bottom. A hard blow might cause the hammer to spring enough to let a firing pin hit a primer, or in rare cases the stud on which the hammer revolved might break, and then a blow on the hammer would fire a cartridge that was in front of the firing pin.
While accidents related to the above were highly uncommon, S&W realized the potential, and they were further spurred on by Colt’s advertising, that after about 1908 pointed out that their revolvers not only had a rebound feature similar to that in Smith & Wesson’s, but also a second, independent safety that blocked the hammer just under the firing pin. They referred to it as a “positive safety” that Smith & Wesson’s didn’t have.
Smith & Wesson responded by installing safety #2, a hammer block in the sideplate, that was operated by the hand. But as previous posts have noted, dirt or heavy grease could defeat it. The block was also on the delicate side, and sometimes cracked and became non-functional. Also this second safety was not installed in all models, in particular some early N-frames such as the Model 1917.
When the Navy suffered an accident in 1944, known as the “battleship incident,” S&W was pressed to do something, and too do it quickly! To their credit they did, by adapting a Colt’s style safety to their own lockwork. The result was so perfect that the same safety is still used today.
From a personal point of view, I greatly enjoy older S&W revolvers because of the wonderful craftsmanship they represent. But I seldom carry one as a weapon that doesn’t have the current style hammer block (used in most models from 1945 to present). I have found that there is little risk in carrying 5 rounds and the hammer down on the empty chamber while having serious shoot-outs with tin cans and paper targets. I began the 5-round technique when I started carrying Colt Single Action revolvers, and soon found myself doing the same with other revolvers carried for non-defensive purposes. Over a long lifetime nothing ever caused me to change my mind, and there is no safer way to carry a revolver, regardless of when it was made, or who made it.
Others of course can do it differently.