The rear sight on your 617 is a Millett which was very desirable with the bullseye guys. They are no longer made and finding them is hit and miss, mostly miss, and a tad expensive when you manage to hit. I have them on both of my no-dash 617s as well as several other S&W revolvers. A replacement factory sight shouldn't be hard to find if you want the original part to complete the package.

Ed