I believe the 1860 grip is a direct fit to the Colt SAA and its exact copies. I am not aware of any original Colts that were so modified by the factory, but that is not to say it was never done.
According to Cochran (colt peacemaker encyclopedia)
The standard peacemaker has navy-length grips, and the "extra long " variation has the the army-length grips carried over from the 1860 colt army.
Cochran also states, long grips model peacemakers (1st generation) are extremely rare. Maybe 50 known. Replaced by the colt bisley.
