cyclehealth

Could someone tell me what the small part immediately aft of the rear axle is called so I could purchase for the one that is missing on the left side of the bike? It appears to act as an axle stop when setting the rear wheel in it's place within the rear dropouts.

Thanks!

P.S. Bike is 87 Schwinn Sierra
The part in question is the small silver part with a screw through the center of it, located at the rear most section of the axle dropout. I am missing the one at the left side dropout and would like to replace.