I recently put my brakes back together after a new pad and resurface rotor install and was missing this piece on the passenger side. It is a small spacer that is on the spindle shaft. Without it, the hub attached and rotated fine but the caliper was off by about 1/8". The previous owner or mechanic must have pulled it off with the hub and not noticed it missing. Not sure how the brakes ever worked as well as they did without it.
