You want a #8-32 x 1/2" set screw for all S&W revos. All the ones I've done, anyway. Check
McMaster-Carr for all fastener needs.
I like the black carbon steel ones best. The stainless ones are softer and the hex gets stripped out easier on them. Use blue LocTite to secure them and you will never have any rust problems, as the Loctite seals the threads. I get the flat point or dog point ones.