First, I take off all 3 pieces of the "left hand dust cap unit" (27). Then I can simply put the rollebrake there, but there is a gap between the brake and the hub. My guess that this gap is supposed to be filled with the "seal ring for brake" (26), but I didn't find this part. Should it have been included with the hub or the brake, or is it something I have to order separately?
I sold my bike with the Nexus 8 earlier this year, I don't remember exactly that particular part. But I think it should have been supplied with the hub + rollerbrake.
Incidentally, I am in the process of building up a SS with a Shimano rollerbrake and Nexave freehub (which has the splines on the left side, to accomodate any Shimano rollerbrake) and there is no such ring.