I don't Carl, just what I read in an article on what they did. I've seen a photo of the front spar extension though, which if I understand it correctly, was used on every other Sunderland afterwards. It was just a short length of spar with the bolt connections at both ends.
On the kit, the rear spar is easy to identify, it's the panel line on the wing that corresponds with the rear end of the kit's location tab, front end of that tab happens to be where the front spar is.
If you do a google image search for Short Sunderland prototype, there's a few of it in the first few pics. A bit further down is another which says was a German Recognition manual showing the prototype
EDIT: Just looked on the What-If forum to find my thread there on my project, only I haven't started one, just added to an on going one. The wing was rotated 4 degrees backwards at the rear spar point where it attaches to the fuselage, so I cut a 4 degree wedge shape slot in the front of the spar then added a four degree wedge behind it.
IIRC, this makes the rear spar perpendicular to the fuselage center line. wrong, it makes the rear spar 2 degrees forward of perpendicular instead of 2 degrees backwards as the production Sunderlands were.
EDIT 2: The thread I thought I had on the What-If forum isn't there because ----- it's on this forum
http://beyondthesprues.com/Forum/index.php?topic=1235.0