Smith & Wesson did indeed "blue" some of their 66s and I believe your model is a stainless gun with some sort of a blue finish applied. I've often wondered what they looked like. It the blue highly reflective like Smith's royal blue, or is it matted in any way?
I remember back when gun magazines were still writing about double action revolvers a lot and talking about blue v. stainless (a topic they kept alive for years with banner headers). Pundits griped about stainless as being too reflective. I don't think too many of them took them into a darkened room to actually find out. (I did and my stainless Security-Six was about the same as my blued one.) Still, the argument went on for some time.
Leave your 66/19 on the lawn under a water sprinkler for a weekend. If it rusts, it's a 19. If it doesn't, well, you'll know it's stainless steel. Actually, a better test would be to take a cotton swab and put just a drop of cold blue solution on it. Clean and apply that drop to a hidden part of the gun and if it just beads up and doesn't change color, then you know it's stainless.