Once, when I was installing a spur hammer on a 5906, the hammer would not hold in single action.
Closer inspection revealed damaged single action notches on the hammer. (Donor gun the hammer came from was probably dropped while cocked and that broke the notches.)
Company I purchased the hammer from apologized and sent me a new hammer and told me to keep the old one.
I installed the replacement hammer and all was fine.
I then ground off the damaged single action notches on the bad hammer, removed the thumb spur, and shaped it into a spurless hammer for a DAO conversion.
Win-win for everybody.
John