What does the "D/V" marking on small DC motors mean?

D/V most likely stands for disk varistor as Vicente Cunha suggested. Not only does the Mabuchi Motor website use the term D/V in this way, 5.9V and 13.0V also match up with two of the standard threshold voltages that TDK produce ring varistors for motors in, and these photos of another brand of CD spindle motor appear to show one of them soldered next to the commutator. (The black ring with solder pads around the outside. Apparently it needs to be on the armature to most effectively suppress noise, hence why companies like TDK offer specially-shaped varistors for this purpose.) So in short, D/V 5.9 probably means that it contains a varistor with a threshold voltage of at least 5.9 volts but possibly more due to manufacturing tolerances.