Leonard Francolini is not only a world class engraver but has a long and close association with the Colt Firearms Company.
Colt made screw less versions of the SAA revolver for special presentations.
I expect this magnificent pistol was a special order for Leonard's wonderful engraving.
Leonard has been modifying the frames to screwless on guns he has been engraving for the past few years. If you look on Leonard's web site, there is a series of pictures on the right side that shows his process. http://www.francolini.com/