Your most affordable alternative would be a 19-5. In 19s the dash 5 means elimination of recessed cylinders and barrel pins. Unwanted shooter grade 6" 19-5s are fairly common. My local gun stores price them at $425 plus tax.

6" 66s are a lot less common so they cost more. They were not a production item until after the silver colored stainless rear sights were discontinued so that is not an issue. Recessed chambers add a cleaning chore without adding anything of value to shooters and barrel pins were non-functional so let collectors pay the premium for P&R. Prior to the current 66-8 there was never an endurance package in K frames. Dash 3 improved the front side plate screw to the modern larger head screw with a spring loaded plunger keeping the yoke from moving forward in the frame. The next significant changes for shooters were drilling and tapping the top strap for optics which was accompanied by the dash number increasing to 4, then elimination of square butts during dash 4 production. Neither were good or bad for everyone. They are relevant if will you want to mount optics or like traditional square butts. Next came an important change that is seldom mentioned in collector books. Approximately 1997 square cornered lands and grooves cut with broachs were superseded by rounded corner lands and grooves made by eroding away metal with electricity. Whether it is as easy to find accurate lead bullet loads with the modern rifling remains controversial. Are you getting good results with cast bullets in your 620 and other modern S&Ws or will you only shoot jacketed bullets?

I hope this helped you pick a grail 6" 66.