Thanks for your input. The adjustable rear sight is milled at the factory quite differently than the factory Novak low mount fixed sights and they are not interchangeable.
The factory fixed sights have the slide milled at the factory for clearance, as do the factory novak night sites. What is unclear to me is the cut in the slide the same for both the standard fixed and the night sight fixed.
I had looked at the Novak site, couldn't find dimensions for the cuts between the standard and the night sights. Called Smith and Weson and after a fairly long discussion, the tech said "they should be the same, but I'm not sure".
The reason I care is that I'm thinking of ordering a 5906 with factory night sites, but they are "dim", so I was hoping I could replace them with standard low mounts.
Jim