The stud on the back side of the hand above the pivot pin is a safety feature (added engineering change) that intercedes in front of the forward shoulder of the bolt (between the bolt and the back side of the breechface) and prevents the thumbpiece/bolt assembly from moving forward after the action has been cocked, and during firing (under recoil), when the trigger is pulled. Along with the rear leg on the bolt, it also prevents the hammer from being cocked after the cylinder has been opened.

I'm not sure of the exact dates concerning the changes, but I believe this change was first incorporated into the "magnum" caliber revolvers.

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Ret. LE, FA Instr, S&W Armorer