Hi Ron,
The B5 superheaters are what is known as coaxial type. They consist of a 1/2" diameter stainless outer tube with a 5/16" diameter copper tube running down the middle. The outer tube is blanked off at the firebox end and the copper tube doesn't quite reach the end. The steam from the wet header travels down the stainless outer tube and returns up the inner copper tube to the dry header.
Frankly, the coaxial type of superheaters are not very efficient and you would do better replacing them with the conventional type with two separate tubes jojned at the firebox end, the sort you are used to. Extend them over the fire right to the back of the firebox and you will get superheat worth having.
Here's a cross section of a coaxial type that I drew up in 3D for Neville Evans's Didcot that I've got all the castings for.

This is slightly different in that the end of the inner copper tube is supported by the blanking plug in the end of the outer stainless tube and the steam enters via a slot in the end. I won't be fitting one of these but the normal two element type instead.
John
Edited By John Baguley on 20/07/2015 18:20:29