Having issues with the Bolt Stop Cross Pin walking out of your MK IV frame during heavy use? The Volquartsen V-Lock V-Lock Retainer Spring provides positive retention of the pin so that it can't drift out on it's own, but without impacting your ability to take apart the pistol. NotesReplaces part # 41 on the Ruger MK IV and part # 37 on the Ruger MK IV 22/45The hammer will need to be removed from your pistol in order to install this part