The 6 inch barrel option didn't come along until 1978, during the dash 1 production, and the loss of pinned and recessed began in 1982 at the dash 2 production. The gas ring problem mentioned by Texas Star had been fixed by the intro of the 6 inch barrel..

I bought 2 of them when I was first hired as a peace officer, the first one in 1981 and the second early in 1983. One had a white outline rear sight, the other didn't so I added one.

It seemed that very few of the Model 66's shipped with Target hammers and triggers, especially if compared to 6 inch Model 19's. I added the wide, smooth "Combat" trigger to mine. The target hammers and triggers used on stainless guns at the time are just flash chromed for appearance sake. Loose target triggers seem easier to find in silver finish than do hammers, but both can be found with some looking. Lots of folks swapped them into their guns.

The MIM trigger began towards the end of the 66-4 run and the MIM hammer was added at the 66-5 mark.

Looks like shinbone should be looking for a 66-1 through 66-4 gun.

As a rookie cop and gun nut/S&W enthusiast, I tried out a lot of guns, K, L and N framers, 3-1/2 to 6 inch barrels and always came back to the 6 inch M-66. It seemed to provide me the optimum in controllability and reduced muzzle flash while not feeling like it was adding any extra weight on my belt over a 4 inch gun.