Again these differ between the models the parts book does not list a part for the RB but I have RC parts fitted to my RB and they work just fine. I assume you are looking for the RC part.
RC 90104-MA3-660 Bolt end piece
RD 90104-MA3-850 Bolts stand
If you look at the part the RC part has the Hexagon-Bobbin-Thread. The RD has Bobbin-Hexagon-Thread.
I have an RC set which are waiting to go for replating,suppose it was a long shot to get hold of a set for the RB also,CMS and David Silver are showing discontinued,may be a job for a man with a lathe