Hello together
The two models are identical, in the past these models had a 3-fold price difference, presumably this was or is OM packaging. It is important to note that you can only fit this to hubs with a screw-on sprocket. A sprocket puller and possibly a tube (torque extender) are required to remove the old sprocket. Over the years, the screw-on sprockets have become extremely bent or tightened. As a rule, children's and youth bicycles still have screw-on rim mounts. You can then tighten them again as normal, the direction of rotation of the sprockets is again the direction for tightening. Greetings and have fun.

The smallest gear ratio is different, largest sprocket
MF-TZ500 34 teeth Attention: longer change and longer chain may be necessary to be able to change gears
MF-TZ21 28 teeth