To complicate matters a bit, I have a 686-6 6" and a 686SSR 4". The SSR does not have a full underlug barrel -- it has sort of a semi-full underlug arrel that tapers from the front end of the extractor rod towards the end of the barrel -- and weighs 36 ounces, compared to the 686's 44 ounces. I hadn't thought about it when I bought it but, in reality, it comes closer in physical specs. to the 620 than to the 686. Yet, when you look at the model series # under the crane it reads "686-6". Go figure.
I think both guns are fantastic and yes, they have 2-piece barrels, are not pinned and recessed, have locks, and have MIM parts. I can find absolutely no difference in quality between these guns and the older Smiths that I own.