it's the other half of the track nut im guessing.

it snapped on me yesterday and im having a little bit of trouble describin to the LBS on the phone to see if they have it in stock before i head out there.

axle end caps

thanks boss.

anytime. sweet ass bob jackson, btw.

That is the lock nut, I think.
I had mine snap on my a couple weeks ago.

https://www.sheldonbrown.com/cone-adjustment.html
Also the threading on the axles varies from front to rear hubs and brands.
Tell them you need a Lock nut for a Formula Rear Hub

fantastic. thanks gentlemen.