If you have a vernier caliper, measure the diameter of the shaft of
the thumb wheel. Armed with this diameter in millimeters, go to

http://www.rotorclip.com/products.php

You want one of the "Axially Assembled External Metric" clips, most
likely the DSH designation. Click on the DSH link and you will get a
table that gives you all the sizes for this type of clip. Compare
these to your thumb wheel sizes to determine the correct clip.

Good Luck!