A Weaver #62 rail is what S&W finally told me is what they used to determine mounting hole spacing but you can't find a #62 rail anywhere. Weaver/Bushnell advised that #62 is a wholesale identifier and won't work on the retail level - there you need to look for a #48062. I installed one after shortening it yesterday; it fits the curvature of the top of the slide and its mounting holes perfectly.

Ed