Howdy
Smith and Wesson has never put Transfer Bars in their revolvers. Ruger uses Transfer Bars. In this photo, the arrow points to the transfer bar in a Ruger New Vaquero. The Transfer Bar is a device that transfers the impact of the hammer to the frame mounted firing pin. The Transfer Bar only rises up to transfer the blow when the trigger is pulled. If the gun should fall on the hammer spur, and the sear breaks, without the Transfer Bar raised, the hammer cannot reach the firing pin and the firearm will not fire.
Current Smith and Wesson revolvers use a frame mounted firing pin that is struck by the hammer, however S&W has never used Transfer Bars.
Instead, S&W uses a Hammer Block to prevent inadvertent discharges. The Hammer Block works exactly the opposite from a Transfer Bar. The Hammer Block prevents the hammer from falling far enough to fire a cartridge unless the trigger is held back. S&W has been using Hammer Blocks for many, many years. In this photo, the long, thin, slanted piece is the Hammer Block. Notice how the tip of it sits between the hammer and the frame. If something inside the revolver should break, the Hammer Block will prevent the hammer from moving forward enough to fire a cartridge. S&W first began putting this style of Hammer Block in their revolvers around 1944. Previous to that there were at least two different designs of Hammer Blocks in S&W revolvers.
Here is a more recent revolver with the lock and MIM parts. The Hammer Block in this revolver works exactly the same as in the older revolver.
Model 36.
This Flat Latch was made in 1961.
This one shipped in 1969.