We've provided this page as a handy reference for identifying the major differences between the 1st and 2nd generation peacemaker revolvers. These are mainly cosmetic in nature:
1st generation frames are fitted with a cylinder pin retainer screw, which must be unscrewed before the cylinder pin can be removed. 2nd generation frames are equipped with a spring-loaded cylinder pin retainer pin which is pressed to remove the cylinder pin.
Pietta's 1st generation revolvers come with brass backstraps and trigger guards (much like most of their percussion aka. 'cap and ball' revolvers), while the 2nd generation single action army revolvers all have steel backstraps and trigger guards. All Pietta 1873 SAA revolvers have steel frames regardless. The 1st generation revolver's grip profile is slightly different as well.
Pietta's 1st generation revolvers have the narrow early type 'pinched' rear sight groove on the top strap, while the 2nd generation peacemakers revolvers have a wider rear sight groove.
Pietta's 1st generation revolvers have the early type 'bullseye' ejector head (the button on the end of the ejector rod), while the 2nd generation peacemakers revolvers have the low profile crescent-type version..