This is the Gen2 magazine for guns with a -KM part number which have a Keymod rail. These have a FDE follower. Please check the original magazine that shipped with your MPX. If it has a FDE follower then this is the magazine you need. If you have a black follower you need Part Number: MAG-MPX-9-30. We cannot guarantee 100% reliability when using these magazines in the "Gen 1" MPX.