When I was younger the longer barrels where more popular but the trend towards more portability has relegated the 6" and 8 & 3/8's tubes more to those of us who are moss backs . . .
Another "mossback" checking in.

I have a tale of two 6-inch Model 66s. I was on the hunt for one for years but rarely saw one in Indiana. Despairing of that, I found one in a periodical and ordered it from a college professor-seller down south. The gun came in and was in nice condition but had been polished a bit. That was a minor problem - nothing was harmed. I sent a note telling him all was well, but then when I fired it, it would not stay on a B33 target paper at 15-yards. I couldn't believe my eyes.
I called the fellow and he very graciously offered to give my money back, so the gun returned to Georgia. The search continued, and finally another one was located at a dealer in northern Indiana. Leery of it from previous experience, I was anxious to test it. This one was the opposite number from a parallel universe. The first six rounds out of the gun at 15-yards made the proverbial "ragged hole." The next six, at 25-yards, went in a group small enough that I will not mention the actual size, but I did keep the target (for my own amusement). Federal American Eagle 158gr. JSP cartridges.
Since then, that gun has never failed to amaze. It shoots way better than I can. It will be one of my last .357s to go.
I take it that gun shown is an original Model 66... ? Very nice. I don't think I have ever seen one of those. My gun is a -3 version. Seeing your gun does make one wonder how many of the early 66s were made with the long barrel.
Editing in a note to my already long-winded reply: In the course of looking for a 6-inch 66, I did come across a couple of commemorative Indiana State Police guns (don't recall the vintage, but probably newer than your original 66). I always try to stay away from specially marked guns anyway, but found this particularly odd because I have lived here all my life and don't recall ever seeing a state trooper carrying a 6-inch revolver. I always wondered how that deal came to pass... ?