Most single actions had a safety notch in the hammer, but it was not strong enough to keep the gun from firing if dropped on the hammer, so many folks didn't depend on it and carried the gun with an empty chamber or a fired cartridge under the hammer. When double action came along, the ones with side swing cylinders had a rebounding hammer that was pulled back from the primer so the cylinder could be swung out. It was soon realized that the rebound mechanism itself was not strong enough to prevent firing if the gun was dropped on the hammer, so the rebounding hammer was combined with a mechanism that either blocked the hammer so it cold not move forward even if struck, or another part filled a gap to transfer energy.
One of the first, and still one of the best, was the IJ system which uses a hammer that cannot reach the frame-mounted firing pin unless a part, called the transfer bar, is imposed between them to transfer the energy from the hammer to the firing pin. Ruger uses that system, but S&W and Colt use a hammer block that is moved out of the way of the hammer when the trigger is pulled.
Jim
P.S. In 1960 or thereabouts, gas was as low as $.17 a gallon for regular, $.21 for high test. That was high; the fixed price during WWII was $.09 a gallon for regular; high test was generally not available, almost all supplies going to fuel aircraft, but then few automobile engines needed it..
JK