Thank you everybody for your input. I appreciate your help. This forum is a fantastic resource.
The cam has a pointed 4-40 set screw in it AND is tight on the shaft. We didn't try real hard, but couldn't move it a bit. We'll keep an eye on it and make sure the set screw stays tight.
We used a milling machine as a drill press to drill a #42 hole into the shaft hole of the thumb lever. Then, mounting a tap in the milling machine, started (by hand) the threads for a 4-40 set screw. Then we mounted the thumb lever on the shaft, set the position and marked the point on the shaft where a pointed set screw would sit. Then we dimpled the shaft at the proper point. We checked the alignment of everything and then blue Lock Tite on the 4-40 pointed set screw and we were done.
Admittedly it took a couple of hours but we were talking through each step, double checking measurements, diameter of the shaft, length of the turned down portion of the shaft, depth of the hole in the thumb lever, diameter of the bit, etc., etc., etc. You name it, we talked about it. The only thing we talked about but forgot to do was to put a little red Lock Tite on the shaft before friction fitting the thumb lever back on and installing the set screw. Don't think that will make much difference and it would have made it more difficult to remove the thumb lever if necessary in the future.
Again, thank you everyone for your input. Much Appreciated. Best Wishes for a Happy and Prosperous New Year!!