Brake or tachogenerator?

[Erich Müller <emcuser-at-gonzales.swb.de>]

Hello,

got a stepper motor that has bolted to its rear axle a part that has a
commutator and looks like a permanent magnet motor. Not as big as the
stepper still substantioal in size(maybe 1/3 of the size of the Stepper)
Is this a tacho generator or brake or what else could it be? What use
would a stepper and DC-motor or tacho generator/stepper combo have? Fast
homeing?  Or is it just a brake/locking device for the axle? The label
does not give any info about the usage of the bolted on component even
though the rotor of the stepper and DC-motor (if it is one) share the same
axle. 

Thanks,
Erich