I ordered this mouse (trackball) and the model I received was FDM-G51 P (with a green ball, not as shown).It lasted a good hour or so, then the vertical roller stopped working. There's a very weak spring that holds both rollers against the ball, and there's more friction in the lube than there is between the ball and roller.Some models might be better than others, but if you can't trust that you'll get the right one, just stay away from anything that looks like this.