Here's a picture of what's on my 5-1/2" version of that vise.
It's a strong spring (to stop backlash), a washer, and then an E-clip.
The clip would go in that first groove just before the threads.
Check SearsPartsDirect.com to see if the parts are still available. If not, the collar should get it working again. You should be able to find one in the parts bins of a good hardware store.