One clarification on Gary's post here:

If you take the sideplate off and remove the hammer block and install the sideplate, the revolver will fire. If the hammer block falls out and you manage to put it in wrong and get the sideplate back on with the hammer block installed wrong, the gun may not fire. But it will fire for sure without the hammer block; you are just removing a safety device that S&W added to their revolvers shortly before or after WWII after an unintentional discharge happened to a military person (maybe a Navy guy) had an AD in the 30's I believe.

I actually ran across a revolver that a friend owned that had the hammer block removed by a "Bubba gunsmith". It was a S&W model 17 I think and I had taken the sideplate off to clean the internals and saw the hammer block was missing. I ordered the hammer block from Numrich and when it came in, installed it on his revolver. Bubba had also cut a few coils off the rebound slide spring too and the pistol was having slow and hesitant movement of the trigger back to battery and was the original reason I took the sideplate off because I thought it was gunked or rusted up internally. I replace the rebound slide spring too on his gun.