Spangenberger's probably right.
I always associated Stag with post-WWII production - reaching a peak during the heyday of the Western movies and TV shows, since it was dramatic.
Colt offered Ivory, Mother-of-Pearl, and different woods, as well as Hard Rubber during the Frontier era, as well as varying grades of engraving.
Colt understood 'marketing'...
Neither Kopec's 'A Study of the Colt Single Action Army Revolver', nor Sutherlands 'The Book of Colt Firearms' show any on Old West era originals.
In Cochran's 'Colt Peacemaker Encyclopedia' - their first mention is in 1952 - as being supplied by the Rocky Mountain Mineral Company, of Casper, Wyoming - for Colt, since Colt sub-contracted them out.
This dovetails with the turn of the century popularity of the horse opera, and all those sets of 'Franzite' grips as replacements for damaged originals that's doll the otherwise-tired piece up a bit.
Vaya,
Scouts Out!