It has been asked in comments why a person would want to remove the single action notch from the hammer of an otherwise perferctly functional DA revolver such as the SW 625 or 646. The reason for doing this is actually two-fold, as there is a mechanical and competitive reason for the modification. The mechanical reason was stated in comments to the previous post, in that by removing the SA notch and polishing that area of the hammer, you can smooth up the last portion of the DA trigger pull just a little bit.
However, the primary reason to entirely remove the ability of the revolver to fire in single action mode is to pass the trigger weight test for NRA Action Pistol production guns. The rules for Bianchi Cup state that the trigger in Production division may not be “less than 3.5 lbs. in any mode”. What that means is that if you have a nice action job on your DA revolver that drops the DA trigger to a slick 8 lbs, the SA mode cannot be less than 3.5 lbs for Production, and 2.5 for Metallic. By removing the SA notch, you ensure that even with the slickest DA trigger on Earth, your gun will still pass the trigger test for Production division and be legal to shoot.