This style replaces the pinned front sight and rear blade.
http://www.meprolight.com/default.asp?catid={0A19467A-BB97-4974-949E-B32AE2F02C84}&details_type=1&itemid={D7C12EAA-DE12-4334-9960-730075D4F404}&tempTyp=1
There is a tiny detent spring and ball in the rear sight screw. Don't lose it when you remove the old screw. It fits in a nearly invisible hole in the screw head. This part is an example of why parts diagrams are called "exploded views." (The other is the rebound spring.)
I installed one of the Meprolight chips in place of the red insert. It is the size of a peppercorn and sticks up and drags on the holster. At the time, there was only a pinned front sight for J-frames.
The front sight is a very tight fit. Insert the new sight fully, then drill halfway from both sides using the existing ramp holes as a guide. Better yet, have a gunsmith do it.