Is this Gen 1 or 2 MPX?
Referenced Product: WMPX-4B-9-KM-SBR
The MPX line doesn't have an official Gen1 / Gen2 designation but the keymod designation is used to identify the equivelant of Gen2. Part numbers with -KM will be the more recent design (as of today, 7-12-18). The -KM mags have gray followers versus black on the first release (equivelant of Gen1).