I have never heard the term "trombone" for S&W stocks...? I can tell you for sure that the OEM for that 686-3 is the Goncalo Alves Target. The hammer and trigger should be a dull, matte silver color.
I might argue that price as slightly high if it has no original box but it's still absolutely within the current market. In other words, if you really want a 686 with a 6-inch barrel, a 686-3 is a fantastic one to get for damn sure and there is no way that $650 is too much to spend if the condition is great and the mechanics are right.
I'd buy it at that price if I didn't already have EXACTLY the same thing.
I believe I paid $315 for mine new in the summer of 1989. I believe retail was around $420 back then. But it's been a few years...