Hi, Mike.
Don't have the Mentor Mill, but do have a Maximat 7L lathe/mill combination.. I also have a copy of the manual. It shows the connection for a 3-phase vertical motor, not to a plug, but to the lathe switch box. Let's see if this helps any.

The 4-conductor cord has the following connections: yellow-green is protective ground, an industry standard color. The brown, blue and black wires go to the hot AC mains. There is a note to change the blue and brown connections if the motor turns the wrong way.

So, even if the colors are not the same as yours, just use an ohm meter to determine the wire connected to the motor frame and you will know the other three wires go to the 3-phase mains.

Let me know if I can be of help.

Paul