According to what I read about S&W history, a U.S. WWII sailor dropped a S&W 38 Victory model on the steel deck of a ship. The gun fired and a sailor was killed. S&W's president was VERY distressed that one of their guns had killed one of our own, and ordered a redesign that included a better safety system. Not because of lawyers and product liability, but because it was the thing to do. Different age, different men.
A hammer-block safety is a steel bar that must be moved OUT of the path of the hammer, so the hammer can move forward and strike the cartridge. This system was put into Colt revolvers in the late 1890's. This system is used in all Colt 'D, E&I, and New Service guns, also all S&W guns, irregardless of whether the firing pin is hammer or frame mounted.
A transfer bar safety is a flat steel bar that must be moved INTO the path of the hammer in order to fire the gun.
The bar transfers the hammer strike to the firing pin.
The Colt 'J, AA, SF, and Anaconda frames all use this system. In fact, all most ALL post 1970's DA revolvers use the transfer bar system.
The hammer-block system CAN fail or be defeated. (Years ago, target shooters routinely removed the safety assembly from Colt revolvers, in the belief that this gave a smoother trigger pull). The transfer bar cannot be defeated by removal, and is about as fool-proof as you can get.