Quoting you Armedeagle because your post is thorough and brought up a lot of points.
It seems to be more like (late) 1974 and coincides with the gas ring being moved TO the yoke and introduction of the 2.5" version (but they didn't start shipping until October, 1975).
You have to remember that applies to all K-frames with target sights and what may be true for a Model 15 or 19 may not apply to a Model 66 or 67.
There are no Model 66's in the 1K and 3K range. The square butt (no dash) Model 66's are in the (K949xxx) 2K, 4K, 7K & 9K's with the (early) 2.5" version in the 6K and 8Ks's.
Very true and also in the other direction, many that originally shipped with a stainless rear sight had them changed to blued.
The front sight can be more of an indicator. The red insert did not become standard until they started shipping with blackened rear sights and although the ramp was pinned to the base and could be changed (or a red ramp could be added later), it wasn't as easy to do as swapping the rear sight.
The end of the stainless sights seems to have occurred in the mid-7K's and coincides with the movement of the gas ring. <7K18xxx should still have stainless sights and the gas ring on the cylinder; >7K47 should have a red ramp, blackened rear sight and the gas ring on the yoke.
Have yet to come across a Model 66 between 7K18xxx~7K47xxx so if you have or come across one, would love to hear about it.