what holds a 68 Cuda wiper knob on ?

Putting my 68 Barracuda dash back together on the bench, My windshield wiper/washer knob is way to sloppy..... And will fall back off . It looks like I'm missing a small piece that snugs it down on the wiper switch .
My grandson had the knob the other day "future motor head :) " and I got it from him and put the knob back on the bench .
I looked everywhere and cant find anything that looks like it would fit it .
Anybody got pics or suggestions ?

Thanks
Donnie