The Glock Modular Optic System (MOS)

The Glock MOS models come from the factory with a slot milled out of the top of the slide just forward of the rear sight dovetail. The slot is just about two inches long and maybe 3/16 inches deep if you remove the cover or mounting plate. The gun comes with a “cover” plate installed which fills this gap completely, so at a glance, the Glock 17 MOS Pistol looks like a regular Glock 17 Gen 4 model handgun, except for the visible seams.
Even the slide serrations extend to the cover, so the feel will be the same as a standard Glock when the cover is installed.

Even the slide serrations extend to the cover of the Glock 17 MOS Pistol, so the feel will be the same as a standard Glock when the cover is installed.
To mount an optic directly to the slide, you simply remove the cover plate and replace it with one of four included adapter plates. These plates are clearly numbered one to four and are cut with posts and holes specific to a variety of optics mounting systems.
Compatible optics (at this time) include:
Plate 1: Doctor
Plate 2: Trijicon
Plate 3: C-More
Plate 4: Leupold

Mounting an Optic
The process of mounting an optic is pretty straightforward. Unlike some other optics ready pistols, there are two levels of fastening stuff together with screws.
First, remove the cover plate over the mounting area using the provided Torx wrench. You’ll notice that the optics adapter plate kit comes with four different plates as mentioned above. It also includes four shorter screws than those that attach the cover plate. You only need two screws, so I presume that two are simply spares. They’re shorter because the mounting plate is only about half as tall as the cover plate. This makes sense because, when installed, the base of the optic will be lower than the top of the slide.