just a follow up, finally found this ...
Safety Stop on Hammer
The Safety Stop is a flat, shelf-like surface on the hammer which functions automatically. It will engage the sear in the unlikely event of primary sear notch failure. This will prevent the hammer from falling fully forward unintentionally and will insure against uncontrolled automatic fire. It also prevents the hammer from striking the firing pin should your hand slip from the slide or hammer while cocking the pistol, provided the hammer is rotated past the Safety Stop. It is located such that when the sear engages the Safety Stop, the sear can be disengaged by squeezing the trigger. It falls because the sear slides over the flat part of the safety stop to release the hammer. This is intentional but will not allow the pistol to fire. It will not fire because the hammer is almost fully forward when it is released from the sear. The Safety Stop is not a manual safety and should not be engaged by hand. Do not carry the firearm while Safety Stop is engaged.