[JSeery]

The retainers do just that, they retain the hub and drum so that it can not come off of the bearing. Such as in your case, you would not have been able to remove the drum until the retainer was removed. There are different types, but a simple one bolts behind the backing plate then extends up and over the backing plate and then curls over the drum. It doesn't touch it, but with a broken axle it prevents the hub from moving out more than a short distance