It’s best to just take it from the horse’s mouth:
Ruger recently discovered that all Mark IV™ pistols (including 22/45™ models) manufactured prior to June 1, 2017 have the potential to discharge unintentionally if the safety is not utilized correctly. In particular, if the trigger is pulled while the safety lever is midway between the “safe” and “fire” positions (that is, the safety is not fully engaged or fully disengaged), then the pistol may not fire when the trigger is pulled. However, if the trigger is released and the safety lever is then moved from the mid position to the “fire” position, the pistol may fire at that time.
Mark I, Mark II™ and Mark III™ Pistols are not affected by this recall
Until your Mark IV™ pistol has been retrofitted or you verify that it is not subject to the recall, we strongly recommend that you not use your pistol.
How To Determine If Your Pistol Needs The Retrofit
All Mark IV™ pistols produced prior to June 1, 2017 are potentially affected and therefore are being recalled. This includes Mark IV™ Target, Hunter, Competition, 22/45™, 22/45™ Lite and 22/45™ Tactical models. These models bear serial numbers beginning with “401” (2017 models) or “WBR” (2016 models).
Firearms NOT subject to the Recall
Newly manufactured Mark IV™ pistols will begin with serial number “500.” Thus, if you have a Mark IV™ or 22/45™ pistol with a serial number beginning with the number “5,” your pistol is not subject to the recall.
Firearms That Have Been Retrofitted Already
Mark IV™ and 22/45™ pistols retrofitted with the updated safety mechanism are easily identified by the letter “S” in the white safety dot that is visible when the safety is engaged.
The Ruger website also provides a form with which you can check your firearm’s serial number against those matching the recall.