Brad:
A bit of interpretation, please.
In your post, you mention top rack...are you looking for the horizontal rack upon which mounts the J1 Box and 4 Way Box, or are you referring to the horizontal metal rack which was part of the Instructional Set ( housing the ATU, with Ae mounted atop)?
Also, "B" set
: There is no such thing as a "B" Set on a RS C42 Station.
Perhaps you might be thinking of a J2 Box, used in RRB?