Yeah.. They're a bit complicated. But I think you just need these 2 to cover your bases. One is thicker than the other to make up for a larger diameter hub. Some times if we're in between, we will trim some of the ribs. Some times we use Hercules Griiiip
if I feel the seal isn't super tight.
I just about punched some kid in the face at Ferguson the other day trying to order something at the will call desk and he wouldn't enter my order unless i had a product number. The part I wanted was Right in front of him...