Magic Chef cooktop knob

I have a magic chef cooktop. One of the white burner knobs got cracked.
I located the knob on your web page and purchased it.
7737P029-60
Stamped on the back of my knob is KIP5687 which is the same number
stamped on the back of the one I purchased from you. The shaft on the bottom of the knob that slides over the rheostat is flush with the bottom of the of the knob that I bought from you and the one I have sticks down about
a half a inch. So, the knob will not stay on the rheostat. I think they make extensions for the rheostat shafts? Is this true or do you have any ideas how to solve this problem.
Thanks Larry