I didn't know there were more then one. Anyway I got one for my 642. It came with grips and the light (laser) is in the grip and points out right under the cylinder. The button is on the front of the grip and lights up when you grip the revolver.
I really like it. Good for pin point shooting.
The thing I like most is dry firing using the laser. You can sure work on your trigger control using that dot. How it jumps around tells you what you are doing wrong.
Sorry, I don't know the model number, but its a Crimson Trace. If something happened to it I'd get right on the net and order another one. I had mine a long time and never had any problems with it.