There is no hammer block in the 642. (or any "shrouded" hammer model) There is no pin on the rebound slide to drive a traditional, separate hammer block.
The primary safety in these shrouded j-frame revolvers is the same as it is in all of the S&W revolvers. The position of the rebound seat on the bottom of the hammer, over the hammer seat on the rebound. At rest, contact between these surfaces holds the hammer away from the firing pin, (or the hammer nose from entering the rear of the breechface) and, prevents the gun from discharging unless the trigger is pulled. No hammer block is needed in these models because of the absence of the exposed hammer spur. This is one of the many reasons why the rebound spring should not be compromised, as it has a critical role in this primary internal safety mechanism.
Photo is of the relationship of the hammer and rebound seats while the action is at rest. (k-frame in photo)