The early "no-dash" revolvers were equipped with natural stainless front & rear sights. I don't think there was a hard date for transition to red-ramp w/ blackened rear sight but it was during the later "no-dash" timeframe. I'm sure there are collectors here that may have some internal memo for the change.
I also have several "no-dash" revolvers. In my own time observations, I never saw a 2.5" or 6" "no-dash" with a stainless rear sight. ALL had a blackened rear and a red-ramp. That certainly does not mean they never left S&W with a stainless rear. By the time the 2.5" & 6" revolvers were produced, I would guess most of the stainless sights had been used up. As noted by others, it was VERY common to find owners/depts replacing stainless rear sights with blackened ones. Depending on the lighting when I shoot, I still spritz a little black on my sights. Comes off in the cleanup.
Anything is possible. That said, I also never saw a "-1" WITHOUT a blackened rear sight AND a colored ramp front. I remember some 6" 66's with a black Patridge front. I don't thing those were ever cataloged but more likely gunsmith-done for the local PPC guys.
Just my .02. Much more knowledge out there than I possess so I'm always a "student"