Don,t forget that when looking at the great photo John has supplied that as you look at them in the pic....that the front edge (in the pic closest to the camera) sits on top of SB2 & that part RS15 needs to be welded along the short edge ( beside the shock bracket in the photo ).
This then all becomes clear ( I hope ) when you turn the assembly the right way up & sit it on top of SB2 .
Hope that helps a little
cheers
andy