Varmint
NES Member
It has to be the RX, the top of a regular P320 slide is rounded. The only way to mount an optic is to mill. The RX is just milled at the factory. There are also companies that will mill after the fact. I think he's referring to the fact that the Glock MOS system consists of a milled slide that accepts different MOS adapter plates, each of which interfaces correctly with the slide and one optic footprint. So though the slide is milled, it is not milled to accept an optic footprint like RMR or Vortex or DeltaPoint, it is milled to accept an adapter plate. You buy (no idea what the gun comes with TBH) a plate for your preferred optic, then all three smush together. The adapter plate lets the optic sit slightly below the top of the rest of the slide, but compared to a setup with no adapter plate (like an aftermarket milled Glock slide or an P320 RX setup), it sits quite a bit higher.
edit: Keep in mind I don't own either an MOS or P320 RX, I'm going off pictures and my decision to get my G19 slide milled aftermarket as opposed to the other two (MOS and RX) that I researched when making my decision. Take my info with a shaker of salt.
Yes that’s what I meant, thanks. My G19 MOS came with a set of 4 plates. I just got the MOS cause the grip has no pinky cutout.