I have trawled through the drawings we are a little closer Drawing number 1260??, lot 215 rings a bell. I will look at work tomorrow, I planned the trip in the lunch breaks and will need to see the email traffic. You can get the drawing copied, there is a cost and you are allowed for private research, so a scratchbuild would be fine. The drawing contained side elevations, plan views front and back elevations as well. The tanks on 1638 I think have been renewed, as has been mentioned, the radius of the tanks looks a little bigger, there is nothing on the drawing giving a rad, the front plating has also been simplified as it should go around the front tank step. John.