Bill,
Try the Novak M1913 1911 rails on Novak's website or Brownells. I put one on a 4566. The S&W dustcover profile is nearly a perfect match. They have them in three flavors: Aluminium, Blue Steel and SS.
Some things you have to be cognizant of: The weight of the rail (and light), The thickness of the dustcover and using 8-40 screws and a knifemaker's epoxy. If you do all of those, you'll have a Hell-for-stout (read: Permanent) light rail.
If you send me your address, I have a blued steel one you can have. It may be a few days before I can drop it in the mail, but it's FREE.
PM me at
[email protected]
Hope this helps!