This thread mentions NIL Grips and has a couple of pictures. There have been many other threads about them. Fine grips. Most people that have them seem to really like them. Have you done a search?
oops I forgot the link. Shows the open back on a 41 Redhawk.
http://www.rugerforum.com/phpBB3/viewtopic.php?f=3&t=229195
Yeah, I only found 4 threads, and mostly just say they look good. I hoped someone might chime in with how they feel, and whether they have the closed or open back design, and how they fit, etc.
Other set's the same. I was putting together another "race gun", bought the Nills slightly used, same sights front and rear... then decided against it. So I put them on this 657 instead.