Well, it is claimed by some S&W folks that the CS1 686's had to meet stringent accuracy and durability requirements.
On another board a fellow who claimed he was a retired customs service inspector told of picking random 3" and 4" CS1's and having to fire "tens of thousands of round through these revolvers.." then checking the accuracy.
If the CS1 failed in any way it was returned to S&W and replaced. I have also heard that the CS1 barrels were air guaged by S&W. I can tell you from personal experience that my 3" CS1 is a 2" gun at 25 yards from a rest.
Free hand standing it is easily more accurate than my 4" 686-4+ . The trigger rivals my PC pre lock 586-5 L-comp.
Another thing that makes the CS1's "special" is that there are not many left in existance. The figure that seems to be most accurate is around 1500 issued 3" CS1's and 1500 4" issued CS1's surviving. The attorney general at the time, Janet Reno, having ordered the remainder to be destroyed, along with many other rare and exqusite federally owned firearms such as the FBI's Colt 38 supers.
There are a few hundred CS1 overuns that were sold to the public during the same timeframe as the Custom Service order. There were also CS3's and "Security Specials" - esentially "civilian market" CS1's - that were produced. But these days a genuine 3" CS1, in it's original box with markings showing customs service issuance, is a scarce and valuable sixgun indeed.