This is a replacement for the little cover that holds the ball in place for the Apple Dekstop Bus II. It's amost exactly the same as the original (the little flanges that lock it into place are slightly longer than they should be) but it seems to work just fine. The original part also has a very smooth polished surface where the ball comes into contact with it; The 3d printed part is, straight off the printer, not so much. I have't noticed any issues with the ball failing to roll, but it might be worth while sanding down or otherwise smoothing out the little sloped part in the inner ring where the ball sits. Printed at .1mm layer height, it might be smooth enough for practical purposes.
Also, please note that there are apparently several versions of the ADB II mouse, and each one has a slightly different ball cover design. I believe this is for the original; it has the information on the bottom of the mouse stamped into plastic. The later ones (at least the other two I have) are stickers instead, and this door will not fit in them.