It was a "Sample" revolver manufactured just prior to the production
run of the SAA. I quote from "New Discoveries":
The first of these two to leave the Factory is the one taken
to the London Agency by General Franklin in mid-June 1872.
Its serial is "S" over "1". The S is believed to stand for sample.
It too is illustrated and the markings in the Book of Colt Firearms
by Sutherland and Wilson as well as in Colt Single Action Army
Revolvers and the London Agency.