Volquartsen Firearms V-Lock Retainer Ruger MK IV and MK IV 22/45. All new V-Lock Retainer 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. Specifications for Volquartsen Firearms V-Lock Retainer Ruger MK IV and Ruger MK IV 22/45: Manufacturer: Volquartsen Firearms Gun Make: Ruger Gun Model: Ruger Mark IV 22/45 Ruger Mark IV Country of Origin: USA Features of Volquartsen Firearms V-Lock Retainer Ruger MK IV and Ruger MK IV 22/45 Replaces part 41 on the Ruger MK IV and part 37 on the Ruger MK IV 22/45; The hammer will need to be removed from your pistol in order to install this part Package Contents: Volquartsen Firearms V-Lock Retainer Ruger MK IV and Ruger MK IV 22/45 What firearm models does the manufacturer say Volquartsen Firearms V-Lock Retainer Ruger MK IV and Ruger MK IV 22/45 is compatible with? The manufacturer suggests that Volquartsen Firearms V-Lock Retainer Ruger MK IV and Ruger MK IV 22/45 is compatible with Ruger Mark IV Ruger Mark IV 22/45Volquartsen Firearms V4LS: V-Lock Retainer Ruger MK IV and MK IV 22/45