Yep that was a ball retainer.
You could dispense with the left retainer but run the risk of the balls dropping into the hub.
I think the left is a standard size. I ran mine for a while without left retainer.
Yeah, "
" sums it up pretty well. They seem to be standard 1/4" balls, no problem there, though I thought seven a rather low number for a rear hub. Did you leave it at 7, or did you put in one or two more?