Originally Posted by
RJ
Mine came with nothing installed, but that was four years ago, and one of the early production models.
I believe that you'll see an M and a L beavertail, as well as an M and a L grip module without the beavertail. Any of the four can be installed. I believe "S" is basically "with nothing installed."
What I do is to get a roll of tape (e.g. duct tape or painters tape), lay the gun facing left. I then use a Glock Armorer's tool (3/16" punch I think it is) to push the rear most plastic pin out. I then attach the selected panel, hooking the bottom first, then squeezing the upper pretty hard to get the holes to line up. I use hand pressure and squeeze the grip hard as I push the pin in with the Glock tool. I turn the tool sideways to the hole, to coax the pin in the final 1/2 mm or so. Turning the tool sideways prevents it from going in too far, as it's caught by the grip once it reaches the right spot in the grip.
There is a second, larger plastic pin that comes hidden on the little "tree" that the panels are stored in the box. Use that one if you install one of those panels, and save the smaller pin, in case you decide to remove the panel again.
I am not a Glock armorer; just done it a few times. I would be glad if someone who knows can post the correct procedure.