what holds a 68 Cuda wiper knob on ?

It's not drilled for a set screw , it does have a widened spot inside the knob close to where the wiper switch shaft shaft inserts that looks like a small piece of spring steel would sit . That would keep tension on it .
I guess this is payback for losing some of my dads stuff when I was young.... LoL :)
I'm gonna search again today for the needle in the haystack.......
If someone has a old ratty knob with one it it , I'm interested

Thanks
Donnie