[vgel]

In addition to the other comments, gears with a prime number of teeth were undesirable because they couldn't be laid out by iterative division of a circle. A gear with 64 teeth can be easily laid out by dividing the circle into fourths, dividing those fourths into fourths, and again to get 64 even divisions. For a gear with a prime number of teeth, the only option is to guess-and-check walk a pair of calipers around the circle, adjusting them iteratively until you make the exact number of steps and wind up at the exact same place. Without vision magnification, this was extremely difficult to do accurately. Clickspring (see top comment) did some experiments with a large dividing plate that makes the process somewhat easier, but it would still be far more difficult than making a non-prime number of teeth.