Sorry about my calling the hammer block a transfer bar...I am not up on all the names for the parts.
I wasn't intending to be a terminology quibbler, and I hope it didn't come across that way. There are transfer bars and there are hammer blocks, but they are functionally entirely different concepts.
A transfer bar is a bit of metal that, upon the right input, is inserted into and completes the physical path by which the kinetic energy of the hammer is transmitted to the firing pin and thus to the primer. An example is the transfer bar in later Ruger SAs.
A hammer block is a bit of metal that, absent the right input, is left in the path the hammer needs to travel to complete energy transfer, so as to block that travel and prevent that transfer.
Remove a transfer bar and the weapon doesn't fire. Remove a hammer block, and it fires just fine.
The interesting thing is that the hammer block in S&W DA revolvers is not part of the original design and does not in fact perform the hammer blocking function. That function is performed by the rebound slide. Smith added the additional hammer block in 1947 in response to the request of one particular customer who had incorrectly diagnosed the cause of one particular event.
You can remove the hammer block from a classic S&W DA revolver, reassemble it, and then perform the "spoon test" (which tests the hammer blocking function) and you will find that it works just fine.
("Spoon test:" cock an unloaded S&W DA revolver, drop a new pencil eraser end first down the barrel with the revolver pointed to the ceiling and pull the trigger. Pencil jumps, signifying that firing pin has hit the pencil's eraser (and would have hit the primer had the revolver been loaded). Now set up the same test, but this time tap the trigger with something (usually a spoon is required) until the hammer drops. (You are simulating a push off, jar off, or sear failure.) The pencil doesn't move! Why? Because in order for the hammer to fall to the point of ignition, the trigger must be pressed and held for the entire time of hammer fall. Why? To keep the top step of the rebound slide from getting under the hammer foot and performing its intended hammer block function.)
For those who are interested, shake a classic S&W DA revolver whose hammer is down and you'll hear a rattle. That rattle is the hammer block.