I am sure Thomas can tell you more, but here are some interesting examples from Stefany's collection that I was able to photograph. First picture shows, like yours, Prosser beads being used as a base for the kiffa decoration.

Second picture shows Prosser beads that have been cut. Perhaps so that they are ready for being decorated to be a Kiffa bead.

I have never seen too many kiffa beads, so I do not know how many actually use Prossers as a base bead. Would be interesting to find out.