As Tbags noted, they're for glare reduction.
Take a look at the attached pic...
This is my 3rd Gen G17 modeling a LaserMax Sabre and Dawson Precision Adj/FO sights.
Immediately obvious is the glare reduction due to the serrations in the flat-black, rear sight.
Also obvious, is how that front FO sight "pops" in that all black rear sight notch!
Last edited by RayB; 06-03-2010 at 14:30..