The correct rear sight blade is a .146 blade. Order the kit from Brownell's or Midway. It includes the W/O blade, windage screw and nut. You will also need a tool to stake the sight nut. You can order it from Brownell's or make your own. The kit is S&W accessory # 191440000. It comes with instructions. You will also need a special (and overpriced) spanner bit to break off the existing staked nut/screw. You can make one if you have an old flat screw driver bit.

The best advice I can provide however is, go to the Brownell web site and view the "How To" video of changing out the sight blade. View it several times before you attempt the task. It is easier, if you know what you are trying to do in each step and how to do it right. If you screw it up, you'll need to order another kit. Also very easy to lose a spring that you will never find if you drop it.