<VV> check ball ?`

[rich dixon]

Hi; can someone tell me on a '66 4-speed trans, is there a check ball that goes in with the back up light switch on the trans? 
It's been three years since I remove this switch from the car I'm restoring,  and today I found the old switch and the little ball that looks like a ball bearing. What is this "ball" for and how does it operate with the backup light switch?
 
Thanks,
 
Rich