The Gobbi Mobile was designed by Gianna Gobbi, a Montessorian and Catholic educator who studied under Maria Montessori.
It is a series of 5 balls, all in the same color but different shades. The balls are hung in a line along a 45 degree angle.The lightest shade is furthest from the baby and the darkest shade is closest.
Most people find that their babies become interested in it around 8-10 weeks.
At this point, babies are supposedly able to start distinguishing different shades. You can see them moving their eyes from ball to ball. This could be because they are distinguishing shades, or practicing adjusting their focus for different distance, or both. If the balls are moving, they may practice object tracking.
This mobile has no angles like the previous two had. A popular Montessori principle is to isolate, or at least pare down, how many concepts are taught at once. So this mobile focuses only on shade and height, not on angles or different colors.