My 640 Pro came with terrific front and rear night sights. If the dovetails match, I'd try to buy the front sight from S & W.

Or maybe have S & W retrofit both front and rear sights. These are easily the best sights I've seen on a snubbie. And the best fixed sights I've seen on any revolver.

See: http://www.gunrunnerstactical.com/im...2022013_03.jpg