Please permit a rather long comment on those pre-war S&W hammer blocks. In the early Hand Ejector models, the only hammer block was the rebound lever, which was powered by a second leaf spring in the grip, in front of the mainspring which was toward the back. It served the initial purpose of drawing the firing pin out of the fired primer so the cylinder could swing out, but provided little protection against a blow on the hammer. Later, S&W developed the rebound slide, a much heavier system which served as a better hammer block as well as rebounding the hammer. The mainspring was moved forward to its current position.
But if the gun were dropped on the hammer, the hammer or its pin could still break and the gun could fire. So S&W developed a true hammer block in the sideplate. The block was part of a flat spring which was loaded to move into the path of the hammer until pushed back into the sideplate by a horizontally moving pin which was shaped as a cam to press back the block. The pin was moved by the hand. It was complex and quite expensive to make and fit.
The next effort was a similar block which was cammed out of the way by a slanted surface on the hand itself; the hand was much wider (front to rear) to provide this surface. This was the system in use at the beginning of WWII.
But note the defect of these systems. They depended on springs to put the safety in the blocking position. A spring failure or dirt could keep the block permanently out of the way of the hammer, and the user would not be aware that the safety was inoperable. They were not "positive" like the Colt "Positive" safety, so much a feature of the Colt revolvers that they named a whole revolver line the "Police Positive". That safety was operated mechanically by the trigger, not by springs, and would not allow the trigger to return unless the hammer block moved into position.
The S&W safety mechanism in use at the beginning of WWII proved deficient. A sailor was killed when he dropped his gun on the hammer. The reports conflict but it would appear the hammer block was frozen into the sideplate and did not spring into position when it should have. Almost literally overnight, Carl Hellstrom designed a new hammer block safety that, like Colt's, was mechanically operated, but by the rebound slide rather than the trigger. S&W has used this safety ever since. Like the Colt "Positive" safety, it will not allow trigger return unless the block is in position, and is sturdy enough to prevent the hammer from moving forward unless the trigger is pulled. Even if the hammer or hammer pin break, the block will prevent the hammer from reaching the cartridge primer.
Jim
