Nope, not an MOS or a slimmed slide. That's a Trijicon RMR cut in the slide. RMR mounting screws sit a little outboard like that, so the cover plates end up with those "ears".
You can tell the slide is not slimmed because the factory serrations are still present along the bottom edge. It is beveled more along the top edges though which makes the cover plate ears stand out more.
Last edited by Yondering; 09/19/17.
